John McEnroe finds it hard to envisage Roger Federer adding to his record 17 grand slam titles and believes the Swiss maestro is showing signs of slowing down.
The 32-year-old Federer heads into next week's US Open seeded seventh, his first time outside the top three in any slam in the past decade, and his recent form offers no suggestion of a quick improvement at Flushing Meadows in New York.
McEnroe, a four-time US Open champion and three-times Wimbledon winner, is not ruling Federer out of contention for the latter stages of the tournament, but feels it may be too much to expect the Swiss to go all the way.
"To me, it's obviously going to be a lot more difficult at this stage," McEnroe, now a television commentator with ESPN, said. "I don't see at this stage him being able to go through all seven [rounds] and have to beat at least two of these [top] three guys.
"Maybe he would use that type of thing as incentive. When you've won 17, you clearly think you can win another one. To me, there comes a point, even as great as Roger has been for so many years, that it catches up to you a little bit."
Beaten by the 116th-ranked Sergiy Stakhovsky in the second round at Wimbledon, Federer also lost to Federico Delbonis in the Hamburg semi-finals and 55th-ranked Daniel Brands on home soil in Gstaad.
He fared a little better in Cincinnati last week, reaching the quarter-finals, where he was beaten by eventual winner and US Open favourite Rafa Nadal.
McEnroe believes that while Federer's experience and ability mean he will still be a factor, grand slam win number 18 may be a step too far now.
With Nadal, Novak Djokovic and Andy Murray all displaying better form and consistency, McEnroe expects tough decisions ahead for Federer, especially if he remains well behind those three in the rankings.
"These guys are hungry," McEnroe said. "There's other guys that want get on the board. He's 32. He's going to have to at some stage decide how bad he wants it if he does dip lower in the world.
"I doubt he'll enjoy being in that spot. All these factors are going to start to come into it. Now, he can shut everyone up if he was able to go all the way at the Open and he could still keep himself in the running."
McEnroe also felt that Federer has lost some of his sharpness and balance. "The balance and the movement are not quite as [Rudolf] Nureyev-like as they were in the past," he said.