From the few interviews I have seen from Rafa this year, he seems to be putting his 'not playing as well as everyone thinks he should' down to getting older, having to work that bit harder, as fit as he is.
I'm sure it must get harder as time goes by, especially if you have had an injury, and don't play as well as you or others think/know you can. I think there are a few sportsmen who go against the grain, Roger is an example, David Ferrer possibly another, who seem to improve with age, and manage to keep playing at top level.
I do worry for some of the young pups coming through, behind the likes of Rafa and Novak, it is so much more than just hitting a ball nowadays, I watch with mouth agape sometimes, as I don't think we really have a clue just how hard it is to hit a ball back over the net from some of the players !
I wonder how the level of fitness can be maintained, and how hard it is on all their bodies.
I love tennis, and am in awe of them all.