The two long-time rivals have had a career renaissance, winning a combined 13 titles and splitting the four Grand Slams between themselves this year.
Nadal won a record-breaking 10th French Open title before winning his third US Open championship as the Spaniard will end the year as the world number one for the first time since 2013.
Meanwhile, Federer started the year off with his first Grand Slam win since 2012 after defeating Nadal to clinch the Australian Open. The Swiss ace followed it up with a record eighth Wimbledon win, and currently sits at number two in the ATP tennis rankings.
Zverev, who faced the duo twice each in 2017, thinks they may have played their best yet, singling out Federer for praise, considering his age.
"I think Roger and Rafa have been playing the best tennis of their lives this year," Zverev said, as quoted by ESPN. "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."
The young German also reserved praise for Nadal, who dominated the clay court season and has looked virtually unstoppable, despite four losses to Federer in the calendar year.
"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."
The trio will feature in the ATP Finals, with Zverev and Federer — who face Marin Cilic and Jack Sock, respectively — playing on Sunday (12 November) and Nadal facing David Goffin on Monday (13 November).