Agnieszka Radwanska claimed her maiden WTA Finals title with a 6-2 4-6 6-3 victory over Petra Kvitova in Singapore.
Poland's Radwanska created history by becoming the first woman to win the season-ending event after completing round-robin play with a record of one victory and two defeats.
In contrast to her semi-final victory over Maria Sharapova, Kvitova made a poor start to the match with a sloppy opening service game as her opponent broke her serve straight off.
Kvitova improved but was unable to break straight back, finally getting on the scoreboard in the third game thanks to a pair of aces.
Radwanska then held serve before two-time Grand Slam winner Kvitova delivered her third double-fault in as many games and she was duly punished by Radwanska as she broke through again.
The No 6 seed continued in confident mood, breezing through her next service game and was able to wrap up a dominant set in 33 minutes with another routine hold.
Radwanska carried that momentum into the first game of the second set, running down everything and breaking the Czech with an unreturnable backhand volley at the net before consolidating her advantage with a solid hold to win her third game on the bounce.
Kvitova stopped the rot to claim her first game of the second set with a forehand winner and she broke three games later with a booming crosscourt forehand winner to make it 3-3 in the second set.
An easy hold put her ahead for the first time in the set and she slowly began to wrestle the advantage away from Radwanska, holding serve and breaking once more to take the final into a deciding set.
Kvitova, struggling with a bandaged right thigh, was trying to keep the points short as she grimaced with pain after each rally as the pair traded a pair of breaks apiece to remain deadlocked at 3-3 in the final set.
Radwanska took control, though, forcing another break before holding to leave her just one game away from the biggest victory of her career, before ending Kvitova's resistance on her first match point when the Czech committed her 53rd unforced error.
"A few weeks ago I didn't even know I would be here," said a tearful Radwanska.
"I was doing the right thing in the important moments of a really close match, I had my chances in the second set but it doesn't really matter how I won."
Meanwhile, in the final of the doubles event, Martina Hingis and Sania Mirza claimed their ninth title of their first season together with a 6-0 6-3 win over Spanish duo Garbine Muguruza and Carla Suarez Navarro.
An emotional Agnieszka Radwanska claimed her maiden WTA Finals title with a roller-coaster 6-2 4-6 6-3 victory over Czech Petra Kvitova in an absorbing encounter at a packed Singapore Indoor Stadium on Sunday.
Playing a first final in her seventh appearance at the season-ending event, the sixth-ranked Pole also created history by becoming the first woman to lift the title after completing round robin play with a 1-2 record.
"A few weeks ago I didn't even know I would be here," a tearful Radwanska said in a courtside interview.
"I was doing the right thing in the important moments of a really close match, I had my chances in the second set but it doesn't really matter how I won," the Pole added.
In stark contrast to her semi-final victory over Maria Sharapova, Kvitova made a lacklustre start to the match with a sloppy opening service game, handing her opponent an immediate break as she got off to the worst possible start.
The Czech appeared a little more animated in the second game and despite working a few openings, poor shot selection and a string of unforced errors enabled the world number six to survive the threat and move 2-0 ahead.
The double Wimbledon champion finally got on the scoreboard in the third game thanks to a pair of aces but unable to make any inroads on the Radwanska serve, the Czech recorded her third double-fault in as many games to cough up yet another break.
Exuding confidence, Radwanska breezed through her next service game and was able to wrap up a dominant set in 33 minutes with another routine hold.
KVITOVA FIGHTS BACK
Radwanska carried that momentum into the first game of the second set, running down everything and breaking the Czech with an unreturnable backhand volley at the net and consolidating her advantage with a solid hold to win her third game in a row.
Kvitova, who continues to battle mononucleosis, stopped the rot to claim her first game of the second set, screeching in delight at a forehand winner as she attempted to stir herself into action.
Her efforts bore fruit three games later when the powerful left-hander forged her first break point of the contest and levelled at 3-3 with a booming crosscourt forehand winner.
An easy hold put her ahead for the first time in the set and suddenly the crowd had a contest to watch after Radwanska's march to victory was halted and Kvitova completed the comeback with a second consecutive break to set up a decider.
Kvitova was trying to keep the points short, grimacing in pain and clutching her bandaged right thigh after each rally as the pair traded a pair of breaks apiece to remain deadlocked at 3-3 in the decider.
Radwanska dug deep once more and forged another break before finally holding to leave her one game away from the biggest victory of her career, ending Kvitova's resistance on her first match point when the Czech committed her 53rd unforced error.
"It means everything to me... in my first final, I get my first win so it couldn't be any better," the 26-year-old Pole added.
In the final of the doubles event, Martina Hingis and Sania Mirza claimed their ninth title of their first season together with a 6-0 6-3 victory over Spain's Garbine Muguruza and Carla Suarez Navarro.
Petra Kvitova of the Czech Republic will meet Poland's Agnieszka Radwanska in the final of the WTA Finals in Singapore on Sunday.
World number five Kvitova, 25, beat five-time Grand Slam winner Maria Sharapova 6-3 7-6 (7-3) in Saturday's second semi-final.
Earlier, Radwanska, 26, overcame Spain's world number three Garbine Muguruza 6-7 (5-7) 6-3 7-5.
It will be Radwanska's first final at the event in her seventh appearance.
The world number six made the decisive break in the 12th game of the third set to finally break the Spaniard's resistance.
"It was a great match from the beginning to the end, lots of ups and downs, so many rallies and a lot of running," Radwanska said.
Radwanska had taken a 3-0 lead in the first-set tie-break before eventually losing it 7-5.
"I didn't know I could come back after that first set - I am just so glad I could win that match," she said.
In the second semi-final, Kvitova broke in the seventh and ninth games to take the first set against Sharapova, who had won all three of her round-robin ties despite arriving in Singapore after a long injury lay-off.
The world number four hit back to take a 5-1 lead in the second set, but 2011 champion Kvitova rediscovered her rhythm to claim the victory in straight sets.
Maria Sharapova has reached the semi-finals of the WTA Finals after inflicting a career-ending defeat on Flavia Pennetta.
The third-seeded Russian comfortably wrapped up a 7-5 6-1 win to qualify from the Red Group, but this result eliminated US Open champion Pennetta, who has announced she will retire after the tournament.
"I've lost the last couple of times against Flavia. I know what a tough competitor and opponent she is. I just wanted to get a little bit of revenge before she goes out of the game!" Sharapova said.
"She's had an incredible career and I was so happy she got the grand slam at the US Open. I was smiling for her."
Sharapova's two-set win also meant that Agnieszka Radwanska advanced from the group after ending Simona Halep's qualification hopes earlier in the day.
Fifth seed Radwanska had lost both of her opening matches, but battled through after a 7-6 (7/5) 6-1 victory over world number two Halep, including overturning a 5-1 deficit in the tie-break.
Her progression then relied solely on Sharapova's result, which sent both of them into the next stage.
"That tie-break was something else and I just managed to make some great points at important moments," Radwanska said after her win.
"It's very nice to hear people say my tennis is entertaining so I hope I can keep hearing people say that over and over again."
Marin Cilic is one win away from clinching his first ATP Tour title of 2015, after reaching the final of the Kremlin Cup with a 6-3 6-1 victory over local boy Evgeny Donskoy.
The Muscovite was no match for last year's US Open champion, who cruised past the Russian in just 77 minutes.
The Croatian top seed broke serve once in the opening set, and three times in the second, as he sealed an easy triumph by winning six games in a row.
Donskoy, the world No 118, has never won on the ATP Tour and he will have to wait a little longer while Cilic will now face No 2 seed Roberto Bautista in Sunday's final.
The Spaniard overcame Philipp Kohlschreiber of Germany 6-4 6-4, with the only breaks of serve arriving in the 10th games of each set.
In a contest dominated by serve, Bautista conceded just two points in five games in the opening set although he faced a much tougher time in the second.
The world No 27 had to save three break points in the opening game but did just enough to fend off the No 4 seed.
Maria Sharapova has been quick to cast doubt over her chances of winning the WTA Finals in Singapore after an injury-ravaged second half of the season.
The 28-year-old enjoyed a positive start to her campaign by reaching the Australian Open final, which she lost to Serena Williams, before her regular season effectively ended with a Wimbledon semi-final defeat to the same opponent.
A leg strain ruled Sharapova out of the US Open and she pulled out of her comeback match in Wuhan with a forearm injury last month, opting to return to Europe to convalesce before arriving in Singapore wary of her chances of victory.
"I've done quite a few start and stops in the last four months, that's kind of been the toughest part, but ultimately, I know that a lot of the girls have been fighting for the last few positions in the last few weeks," said Sharapova.
"I've had the luxury by being somewhat consistent in the first half of the year to give myself the opportunity just to heal, knowing I was already in, doing everything I could not having to rush around, seeing if I could play through a tough injury or not.
"So, yeah, that's been helpful, but obviously frustrating not competing."
When Williams opted to skip the Singapore event after ending her season early following her shock Flushing Meadows semi-final loss to Roberta Vinci, the eight-woman tournament was thrown open but Sharapova does not expect to benefit from the American's absence.
"Look, it's a big step for me to be here and feel that I am healthy," added Sharapova, who will compete round robin play in the Red Group alongside Simona Halep, Agnieszka Radwanska and US Open champion Flavia Pennetta.
"It will be a bigger step that I'll be able to compete in those three matches and finish them off healthy. That's my goal," the world number three said.
"Everything else I just have to take the positives out of because I feel I've committed myself to being here and done everything possible.
"If I can stay healthy and work my way through the matches no matter who I'm playing, in this particular situation I have to worry about myself a little bit more than who I'm playing against."
The five-time Grand Slam tournament champion won the WTA Finals for her first and only time back in 2004 but she does not see herself as the player to beat when the tournament begins at the Singapore Indoor Stadium on Sunday.
"That's not the way that I think. I don't think it would be fair to myself if I thought that way," she admitted. "I'm one of the eight players. I did something to be in the field. I want to take my opportunity and that's how I see it from my side."
Defending champion Anastasia Pavlyuchenkova and former US Open champion Svetlana Kuznetsova are set for an all-Russian clash in the women's final of the Kremlin Cup after contrasting victories in the last four on Friday.
On centre court in Moscow, world No 31 Pavlyuchenkova had to fight from a set down against compatriot and qualifier Daria Kasatkina to finally win 2-6 6-2 6-4 in just over two hours.
Kuznetsova, ranked 32 in the world, had an easier time of it against Ukraine's Lesia Tsurenko, the Russian needing less than 90 minutes to win 6-2 6-4.
Pavlyuchenkova and Kuznetsova may be neck and neck in the rankings, but the latter has exactly twice the number of WTA singles titles to her name, at 14, and boasts two Grand Slam triumphs as well.
There were nearly similar tales for men's seeds Roberto Bautista Agut and Philipp Kohlschreiber in their respective matches, as both progressed into the semi-finals.
Spaniard Bautista Agut beat Lucas Pouille 6-4 6-2, with Kohlschreiber swapping the set scores around for a 6-2 6-4 victory over Robin Haase.
The other semi-final in the men's draw will see top seed Marin Cilic and Evgeny Donskoy face each other.
Both came through their respective quarter-finals in straight sets, with Cilic taking only 54 minutes to register a 6-2 6-1 win over Andrey Kuznetsov.
Donskoy, meanwhile, beat fellow Russian Teymuraz Gabashvili 6-3 6-2 in what was the pair's first-ever ATP World Tour meeting.
Germany's Mona Barthel is on course for a fourth WTA career singles title after booking a semi-final berth at the Luxembourg Open.
World No 55 Barthel came through as an easy winner in her quarter-final match against Mirjana Lucic-Baroni, beating the Croatian 6-3 6-4.
The match lasted an hour and 11 minutes, with Lucic-Baroni broken four times en route, as Barthel set up a last-four clash against wildcard Stefanie Voegele
Voegele continued her good run on Friday, as the Swiss prevailed in the last-eight against Laura Siegemund.
Voegele, ranked 148 in the WTA tour, battled for more than two hours against her German opponent, and eventually won 6-4 2-6 6-1.
Saturday's quarter-finals will see Serbian star and fifth seed Jelena Jankovic take on Japan's Misaki Doi, with seventh seed Barbora Strycova facing Belgium's Alison Van Uytvanck.
Lucie Safarova has taken the final spot in the WTA Finals after Carla Suarez Navarro's defeat in the Kremlin Cup.
After Safarova lost in the first round to Anastasia Pavlyuchenkova on Monday, Suarez Navarro knew she would qualify for the season-ending tournament in Singapore if she reached the final.
But her hopes were dashed when she lost in convincing fashion to Russian Daria Kasatkina 6-1 6-2 in the last eight.
Safarova's season was in danger of a premature end altogether when she suffered a bacterial infection after the US Open.
Instead, the Czech will now be making her debut at the WTA Finals for which she has also qualified for the doubles draw with partner Bethanie Mattek-Sands.
"I am really excited to qualify for the WTA Finals for the first time," said the French Open runner-up. "It has been a dream year and to be one of the eight qualifiers in singles and also compete in the doubles is something that makes me very proud.
"There have already been so many highlights this year, and I hope to finish it playing some more great tennis in front of the amazing Singapore crowd."
Simona Halep, Garbine Muguruza, Maria Sharapova, Petra Kvitova, Agnieszka Radwanska, Angelique Kerber and Flavia Pennetta complete the singles field.
The event gets under way on Sunday with the draw scheduled on Friday.
Anastasia Pavlyuchenkova needed just 84 minutes to collect her first WTA Tour title of the season, as she eased past unseeded German Anna-Lena Friedsam in the final of the Linz Open.
The 24-year-old Russian won 6-4 6-3 after breaking the world No 118's serve four times during the contest.
Friedsam, who was broken in the opening game of the first set, appeared to offer some resistance at the start of the second when she claimed her only break of the afternoon to move 2-0 in front.
But from there, the No 7 seed struck back to win of six of the final seven games and claim her eighth career title on her second match point.