Maybe he's just becoming older and changes his game because of it? Let's not forget his main opponents still are les than 23 years oldBeing a Fed Fan, and watching him thru' the years i feel that ever since his serve started becoming more of a weapon (i.e. in the last one year), he has become more dependent on it, and consequently his allround game has suffered. His slice has become worser, his forehand blows hot or cold, makes passive returns of serve and doesn't run as well as he used to.
I think having a more powerful serve has made him a bit lazy. I hope he gets a coach and comes back with a vengeance next year.