Roger Federer's body is breaking down. You can see it in his movement, which has been noticeably slower ever since his bout with mononucleosis. He has also been dealing with lower back issues since last fall. He is not making his amazing shots as he used to, and is making more unforced errors, which is very unRoger like.
Well, how about Rafael Nadal, who will be turning 23 this coming June? He plays a physical game that can wear him down. It can be seen in his performance at the 2008 U.S. Open against Murray, and he seems to have issues with tendinitis in his knees.
If Nadal keeps winning, he will play more matches. When Federer was dominating from 2004-2007, he played an unbelievable amount of matches. Federer's style is not physical in contrast to Nadal's, and during his dominance, he didn't sustain any serious injuries. Of course, it helps too that Federer has perfect technique.
Can Nadal's body handle it for that long and dominate like Federer did from 2004-2007?