Andy Murray was stunned by Albert Ramos-Vinolas as he threw away a one-set lead in Monte Carlo to lose 6-2 2-6 5-7 in the third round.
Novak Djokovic maintained his command over the ATP Tour with a record-equalling sixth Australian Open title, but can he establish himself as the game's greatest of all time?
Roger Federer commands that status as the holder of more Grand Slam titles than any other male player, six ahead of the Serb and with a mighty 17 to his name.
Six more Grand Slams sounds a stretch for a 28-year-old competing in a golden era of men's tennis, but the reality is that it is turning into Djokovic's era.
The other three of the 'big four' (Federer, Nadal and Murray) have failed to win any of the last five majors, with Djokovic claiming four and Stan Wawrinka landing the other.
Djokovic's latest Grand Slam success has prompted to price up the extent of his dominance in 2016.
The longest odds you'll find surrounding his campaign are for his Melbourne Park triumph to the end of his success, with a complete Grand Slam haul priced at just 7/2.
Such odds may look short for an achievement which has not been managed since Rod Laver in 1969, but that price is justified by the fact that he is strong favourite for all three remaining majors.
A first French Open title is seemingly the toughest test for Djokovic, although it is more of a mental hurdle which requires clearing than proving his superiority on clay, having already dethroned nine-time champion Rafael Nadal in straight sets at Roland Garros last year, only to suffer a surprise loss to Wawrinka in the final.
It is actually Wimbledon give his rivals the greatest chance, the only major event where he is priced at odds-against, with Andy Murray and Federer 4/1 and 5/1 respectively.
If we do see the Serb heading to Flushing Meadows with the three previous majors in the bag then he will be faced with the task of overcoming a field motivated by the prospect of being the man to deny Djokovic history.
To win a first Olympic gold on top of all four majors and 11/1 for him to add a fifth successive World Tour Finals title to that haul.
While these all seem quite realistic possibilities given the way the Serb is dominating the game right now, such success is still a long way off.
However, operating at the peak of his power, 2016 is a crucial campaign for Djokovic if he is to elevate himself from the game's current great to being considered the all-time greatest.
Andy Murray fell to defeat in his second ATP World Tour Finals group match, as an inspired Rafael Nadal wrapped up a comfortable 6-4 6-1 victory at The O2.
The British No 1 raised a few fitness concerns ahead of the upcoming Davis Cup final as he stretched his leg out gingerly after rushing for the ball in the closing stages of a dominant win for Nadal, who took a big stride towards the semi-finals.
Few would have expected the dramatic slump from Murray after he seized a break in the opening game, but Nadal has repaired his fragile confidence during a difficult year and broke back immediately.
Murray strangely used scissors to snip his fringe during an interval, but it was Nadal who offered the cutting edge in many of the prolonged rallies and forced the Scot to repel two break points at 4-4.
The misfiring Murray serve was being continually pressured by Nadal and he finally forged a decisive break at 5-4 up, cushioning a backhand volley to seal the set.
As Nadal resumed his assault in the second set, Murray again fumbled with his serving and a double fault paved the way for another break.
The Spaniard was still not satisfied with his lead, breaking the Murray serve and his remaining resolve to open a commanding 5-1 advantage.
Murray then endured that uncomfortable moment as he appeared to tweak his hamstring while rushing to collect a drop shot.
There would be no sympathy from Nadal, who sealed victory with a fizzing forehand winner, leaving Murray with more work to do in his final match against Stan Wawrinka if he wants to qualify from the group.
Novak Djokovic enjoyed a comfortable 6-1 6-1 win over Kei Nishikori in the ATP World Tour Finals opener at the O2 Arena for his 79th victory of the 2015 season.
It was a 15th straight win for the Serb at the year-ending tournament - a record in the competition's 45-year history.
Prior to the pair's seventh meeting, Djokovic and Nishikori observed a minute's silence as a mark of respect for the victims of the tragic events in Paris on Friday night.
Once under way, it took just 65 minutes to secure an emphatic win as Djokovic got off to the perfect start in the Group Stan Smith.
Djokovic secured an early break in the first set and could have gone 4-0 up, but Nishikori stopped the rot, saving break point with a lovely passing shot down the line before holding serve.
However, the Serb broke once again shortly after. With Nishikori looking to mix up his shots, a back-hand slice failed to make it over the net to gift the Serb a second break for 5-1.
After Djokovic won five successive games to take a 2-0 lead in the second set, the Japanese showed great resolve to hold serve at 2-1.
But two further breaks at 3-1 and 5-1 ensured Djokovic got off to the best possible start in the London tournament, while securing his 23rd successive win this season.
Jamie Murray and John Peers marked their debut at the ATP World Tour Finals with a victory over Australian Open champions Fabio Fognini and Simone Bolelli.
The fourth seeds were in fine form in the opening match of the end-of-season tournament at the O2 Arena in London, holding their nerve to win the deciding match tie-break and clinch a 7-6 (7-5) 3-6 11-9 success.
Briton Murray and his Australian partner looked confident from the off and deservedly took the opening set on a tie-break after seeing a set point go begging in the 10th game.
They recovered from 4-1 down in the second to 4-3 but a mistake from Murray gave the Italian duo another break and they levelled the match.
That left proceedings to be decided by a match tie-break, with the winner the first to 10 points, or by two clear points thereafter.
It was nip and tuck all the way through but a crucial mini-break for 10-9 left Murray serving for victory, and a smash from Peers proved too hot for the Italians to handle.
The victory gives Murray and Peers, who are playing their final tournament together before parting ways, a great chance of reaching the semi-finals.
Murray said: "It was cool. It was a great match, a really exciting match. I think both teams played well. There was a lot of exciting points. It came down to a couple of points right at the end.
"We're really happy to win and really excited to move on in the tournament."
Jamie Murray says he deserves his place at the end of season ATP World Tour Finals in London.
The 29 year-old has enjoyed a remarkable doubles season alongside Australian partner John Peers - reaching both the Wimbledon and US Open grand slam finals.
The pair also won both the Brisbane International and the German Open to end the year ranked as the world's fourth best men's doubles partnership.
17,000 fans are expected to watch Murray and Peers take on Italians Fabio Fognini and Simone Bolelli in the opening match of the Tour Finals at the O2 Arena on Sunday.
And Murray says he is looking forward to playing in front of such a large crowd.
"It's been cool," said the Scot.
"We came yesterday to practise and came in early to hit on the centre court and we did it again this morning. We tried to get used to the court obviously and playing in such a big arena.
"I guess it will be a lot different tomorrow when it is packed out with people watching. We're excited to play, we deserve our place here, we've had a great year and we're really looking forward to getting out on the court and trying to show everyone why we are here."
The Tour Finals will be the last tournament Murray and Peers play together, with Murray instead choosing to join up with Brazilian Bruno Soares for next season.
However, the former Wimbledon mixed doubles champion insists there's no bad blood between himself and Peers, and that both players were happy to move on.
"I guess we've had three good years together, especially this year was great," said Murray.
"I think for both of us, we fancied something a bit different, a bit fresher. It's just life. Life goes on.
"Every decision comes with risk. There's no guarantees for either of us that things will go amazing or get better but that's a risk you take."
Jamie's younger brother, Andy, will also feature at the Finals after qualifying as world number two in the men's singles, behind Novak Djokovic.
Andy, who faces Spaniard David Ferrer in his opening match on Monday, has backed his older brother to impress in London.
"Jamie's obviously had an amazing year," said Murray.
"I'm very proud of everything he's achieved on the court. To have him qualify here - and he did it fairly comfortably as well - he really deserves to be here.
"I was hoping we were going to be able to have one session where he played doubles and I played singles but we're playing on different days unfortunately. I hope he has a good week."
Both Andy and Jamie Murray are expected to be included in the Great Britain squad for the Davis Cup final against Belgium in two weeks' time.
The pair have played doubles together throughout the campaign and Murray says his close relationship with his brother has played a part in the team's success this season.
"We've played two high-pressure matches, which we were able to come through, we've played really well both matches I think," he said.
"It's been fun. I guess we're older. We just go out there and try to do our best for each other and for the country."
British number one Andy Murray will play Stan Wawrinka, Rafael Nadal and David Ferrer in the group stage of the ATP World Tour Finals in London.
World number one Novak Djokovic and seven-time Wimbledon champion Roger Federer are in the other group, along with Tomas Berdych and Kei Nishikori.
In the doubles, Jamie Murray and John Peers have been drawn in the same group as the 16-time Grand Slam champions Mike and Bob Bryan.
The finals start on 15 November.
The tournament is held in a round-robin format before the top two players in each group progress to the semi-finals.
Serbian Djokovic, who secured his 10th title of the year in Paris last week, has won the men's singles for the last three years.
ATP chief executive Chris Kermode confirmed before Thursday's draw that the World Tour Finals will remain at the O2 Arena in London until at least 2018.
Andy Murray scrapped past Frenchman Richard Gasquet to reach his first Paris Masters semi-final on Friday.
The British No 1 won 7-6 (9-7) 3-6 6-3 after outlasting the spirited challenge from the home hope on his home soil in two hours and 38 minutes of gruelling action.
Murray will play Roger Federer's conqueror John Isner or Spaniard David Ferrer for a place in the final on Saturday.
After his win, the British No 1 admitted to stiffness in his lower back, but was pleased to complete the set of reaching the semi-finals of all the Masters 1000 events following his triumph.
"I got the break back at the beginning of the third set but his shot making was exceptional at times.
"He uses the angles maybe better than anyone on the Tour because of the amount of spin his generates. He puts you in positions on the court that not many players are able to. I just managed to fight through in the end.
"I started putting some returns in court, started making my first serves but struggled a little bit to push off. I got a little stiff in my lower back so I was smart with that in the end. Instead of going for my first serves I started slowing them down and dictate points that way.
"It's maybe normal after a couple of weeks break to feel a bit stiff and sore, but that's a good thing so that I can get that out of the system in time for the [ATP] World Tour Finals and Davis Cup. It shouldn't be a problem."
Playing in his sixth quarter-final at the indoor tournament, Murray struggled with his first serve but managed to break the 10th-seeded Gasquet three times to advance in the French capital.
Murray had won his two previous matches in less than one hour but he was made to work hard by Gasquet.
The world No 9 pleased the crowd with some spectacular one-handed backhands and broke in the third game of the deciding set but could not hold his next service game.
Murray made the decisive break in the eighth game then served out the match to love.
World No 1 Novak Djokovic plays Tomas Berdych and 14-times Grand Slam champion Rafa Nadal faces Stan Wawrinka later on Friday.
Andy Murray breezed past over-matched teenager Borna Coric at the Paris Masters on Wednesday.
The British No 1 got his campaign in the French capital off to a perfect start with a 6-1 6-2 victory in the second-round tie.
The 18-year-old Coric is rated as one of tennis' most exciting up-and-comers but his fledgling talents were exposed ruthlessly by Murray.
The Scot broke his serve at the first time of asking, then did so again to seal a lopsided opening set in which Coric appeared overawed.
While the world No 46 from Croatia improved to hold his service twice in the next set, Murray still opened a break advantage and sealed victory in just 58 minutes.
Murray faces a third-round tie against David Goffin - his future Davis Cup opponent when Great Britain play Belgium - on Thursday.
But British No 2 Aljaz Bedene crashed out of the Paris event at the hands of John Isner who won 6-3 7-6.
Elsewhere, Rafael Nadal coasted through a grudge match against his rival Lukas Rosol, beating him 6-2 6-2 in just over an hour.
Stan Wawrinka is into the third round of the Paris Masters after beating Bernard Tomic on Tuesday.
The world No 4 registered a 6-3 7-6 victory in his first ATP Tour match against his young Australian opponent.
Wawrinka got off to the perfect start with a break in the first game which he turned into a 3-0 lead. One further break handed him a straightforward win in the first set.
Tomic admirably fought back but couldn't convert two set points with Wawrinka on serve. The Swiss player eventually forced a tie-break.
Wawrinka then overcame a 5-2 deficit in the second set tie-break, winning six of the last seven points, to book a third round slot against Viktor Troicki or Feliciano Lopez.