Roger Federer is clearly among the best players in men's tennis history, but he doesn't have a clear answer for the Greatest of All Time debate between him, Rafael Nadal and Novak Djokovic.

"How can you compare?" Federer asked, per Howard Fendrich of the Associated Press. "What’s better? To win when you're old or when you're young? I have no idea, you know? Is it better to win on clay or grass? Don't know. Is it better to have super dominant years or come back from injury? I don't know. It really is impossible to grasp."

Federer long held the record for the most Grand Slam singles titles and earned his 20th at the 2018 Australian Open. His two rivals in the Big Three have since passed him, with Nadal currently having 22 and Djokovic sitting at 21.

The 41-year-old Federer announced his retirement last week after struggling with injuries over the past few years.

It helped reignite the arguments over the best ever, which Federer admitted was "a good conversation" and "definitely a fun debate."

All three could have been even more dominant if not for having to face each other during the same era. Twice in his career, Federer won three of the four majors with only a French Open Final loss to Nadal keeping him from the calendar Grand Slam.

Nadal has clearly been the best on clay, winning 14 times at Roland Garros, while Djokovic's nine wins at the Australian Open are also a record. Federer was the most dominant on grass, winning Wimbledon a record eight times.

Federer also had arguably the most impressive run earlier in his career, reaching 10 straight Slam finals while staying No. 1 for 237 straight weeks. Nadal and Djokovic still have enough accomplishments to continue the debate long after all of them retire.

"Everybody calling each other 'GOAT.' 'GOAT.' 'GOAT.' 'GOAT.' 'GOAT.' 'GOAT.' I'm like, Come on, OK? There cannot be possibly that many 'GOATs,'" Federer joked. "In Switzerland, we have a lot of them, but they're in the fields."