John McEnroe has revealed his doubts over whether Roger Federer is capable of winning another Grand Slam title.
The 32-year-old Swiss has not won one of the four major events since 2010.
McEnroe believes the task is only getting harder, although he admits that anything is possible.
"To me, it's obviously going to be a lot more difficult at this stage," said McEnroe.
"I don't see him being about to go through all seven [rounds] and have to beat at least two of these three guys [Andy Murray, Rafael Nadal, Novak Djokovic].
"Andre [Agassi] got to the final of the Open at 35, so there's no reason to believe he couldn't do it but I personally think that it's going to be quite difficult for him to win another one."
Federer begins his US Open campaign on Monday.
