Alexander Zverev makes his Nitto ATP Finals debut this week after a season which has seen two veterans enjoy renaissance years.
The German won Masters events in Montreal and Rome as part of a five-title haul which saw him realising some of his potential, though the grand slams were swept by the grand masters, Roger Federer and Rafael Nadal, who will end the year as world number one.
The 20-year-old said: "I think Roger and Rafa have been playing the best tennis of their lives this year. I really think that, especially Roger.
"He's only lost four matches this year. It's quite amazing how he's able to play at 36 years old; I'm not saying he's old or anything...What he's doing on the tennis court is amazing.
Some had questioned whether Federer, at an advanced age in sporting terms, and Nadal, with his catalogue of injuries, could peak again, but they did in 2017.
Zverev, known as Sascha, added: "Also Rafa, how he came back and played the clay court season was unbelievable. The most he lost in a set in the French Open was three games. "Winning a grand slam losing three games in a set, max, is something that I don't know if we've seen before."
Zverev is ranked third in the world rankings entering the season-ending tournament at the O2 Arena which begins on Sunday. He knows Nadal and Federer cannot continue for ever and is determined to use them as a barometer of his ability.
"All of us are working hard to try to replace them a little bit. This is what we're all working for, to win the biggest tournaments in the world."
He added: "I've won two Masters this year. I'm still working on grand slams.
"At the moment what they're doing on the tennis court is amazing."
Zverev, who plays Marin Cilic in his first match on Sunday, believes he has climbed up a level this year.
He said: "I've had a great season. Winning two Masters is special for any player. The top four has been dominating that level of tournament.
"Nobody has won two Masters in one year for 10 years, outside those big four. That's a great achievement for me."