By Regan E. Doherty

DOHA (Reuters) - There are few more impressive sights in sport than Roger Federer in full flow on a tennis court but the Swiss great cannot compare with Rafael Nadal as an athlete, says former world No.1 John McEnroe.

Debate rages on over who is the better player but McEnroe said Nadal's only rival in terms of athleticism is his old sparring partner Bjorn Borg.

"The two greatest athletes of all time are Bjorn Borg and Rafael Nadal," the American, still playing on the seniors tour as well as commentating, said at a sports conference in Qatar where he was joined by Borg and Romanian Ilie Nastase.

"Federer is one of the greatest players ever, but I don't think he's as good an athlete as those two."

Nadal, who is fit for the season finale after missing the Paris Masters with a shoulder injury, is already confirmed as the year-end world number one after winning three of the year's four grand slams to leave Federer in the shade.

Five-times Wimbledon champion Borg believes the Spaniard could maintain his dominance next season.

"He's still up there. If he stays healthy and motivated, he can sustain it for a long time," the Swede said.

(Reporting by Regan E. Doherty; Editing by Martyn Herman)