Roger Federer and Rafael Nadal enter Sunday's ATP World Tour Finals in the best form of their lives, according to Alexander Zverev.
Both players have enjoyed a renaissance in the past 12 months. The duo, who have accumulated a staggering 35 Slams between them, currently occupy the top two spots in the world rankings after dominating this year's majors.
Federer, 36, claimed the Wimbledon and Australian Open crowns, while Nadal, 31, triumphed at the French Open and US Open. World No. 3 Zverev believes both men are close to reaching their top levels.
"I think Roger and Rafa have been playing the best tennis of their lives this year. I really think that, especially Roger," Zverev 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."
Nadal's resurgence has been just as impressive. The Spaniard entered 2017 as the world No. 9, having not won a Grand Slam since picking up his ninth French Open title in 2014 and being hindered by a series of injuries.
However, six titles this year -- including a landmark 10th victory at Roland Garros and a third US Open triumph -- have seen him rise to the top of the ATP Rankings, where he will end the season.
"Also, Rafa, how he came back and played the clay court season was unbelievable," Zverev added. "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. All of us are working hard to try to replace them a little bit."
Zverev, who is preparing for his ATP World Tour Finals debut against Marin Cilic on Sunday, has won five tournaments in 2017 -- including two Masters titles -- but is still searching for his first Grand Slam crown.
The Press Association contributed to this report.