As fit as he is, I think he's gotten to the age where his body just doesn't bounce back very well without a day off. I think a 3 hour match followed by a day off is easier to recover from than a two hour match without a day off. Just consider how he played in the US Open final. His semi against Djokovic was no marathon (2 1/2 hours), but the short turnaround clearly hindered him. Yet during the French Open, he had a couple of 3 hour matches, and played fine in the next round when he had a day off. So I think that in these one week events, Federer's body might actually be a little fatigued when he's played 2-3 days in a row. Two of his three losses this year came in matches where he didn't have a day off between matches.