World's top two players have defied age to make strong comebacks in 2017, something that is inspiring 20-year-old German
Alexander Zverev wants to learn from Roger Federer and Rafael Nadal
Alexander Zverev makes his Nitto ATP Finals debut this week after a season which has seen two veterans enjoy renaissance years.
Germany's Zverev won the Masters events in Montreal and Rome as part of a five-title haul which saw him realising some of his potential.
The grand slam tournaments were swept by the grand masters. Roger Federer, who is 36, took the Australian Open and Wimbledon titles and Rafael Nadal, 31, claimed the French and US Open crowns to ensure he will end the year as world No 1.
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 is the best of the rest, at 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.
"I think Roger and Rafa have been playing the best tennis of their lives this year. I really think that, especially Roger," Zverev, 20, said. "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.
"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.
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"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.
"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."
With Novak Djokovic and Andy Murray, last year's champion here, out due to injury, there will be a different champion for the first time since 2011, when Federer won for a sixth time.
Zverev, who plays Marin Cilic in his first match on Sunday, believes he has climbed up a level this year.
He added: "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."
The longer-term future of the established elite is uncertain, with age creeping up on them all. Zverev is focused on himself as he seeks to lead the next generation.
He added: "There's a lot of question marks of how they're going to come back. But they're great players. They're always going to be tough opponents.
"This is something I don't think about. I just want to try to keep playing the best I can, try to improve and try to compete in the biggest tournaments. It's as simple as that."