I think he's exhausted. You know, he is "only" 29 but with his style of play he has already in legs more kilometers than Agassi at the age of 36
I think the problem is his head, he stills fit, but he lost his mental edge and the fighting spirit (his biggest strength). Check his past results since late 2006 and he can't buy a close match, he always loses when the match is tight or in the "clutch" (except if he's playing against a mental midget
). When is near to a big change something always happens to him and it's hard to accept many disappointments when you used to be a top 20 player.
About the legs, yeah you're right, his style of play killed him (he has only FH), he missed his speed and he isn't a quick player like he was in 2003-04 after a long break and a surgery in his foot in 2005 never was the same, it's difficult to cover all the court with only one shot (FH) like he did at his prime.