Re: Djokovic's fitness
He's always been very self-protective, i.e. he doesn't shy from retiring if he feels it's bad for his health/body and thereby his future success. This year, he won AO and IW, and got decently far into Dubai. That's a decent amount of matches already, even considering he crashed out in Miami. He's likely to have a lot of matches week after week as he goes from MC to Rome to Hamburg to RG to Queens to Wimbledon. After that there's also a long stretch of matches in Toronto, Cincinnati, Beijing, and New York. So, I think he's trying to maximize his break periods to recharge mentally.
On the whole, I think it's much more mental than physical issues that are in play at this point. He lost in Miami in the 1R not because he was physically spent but because he was mentally tired and lost focus to a great-serving player.
Nole fan: "...most Fedtards seem to be very rabid and rude." (I agree 100%)
You can bet Djokovic's mom just screamed "Another king is dead" right about now - in the remotest corner - of her bathroom - followed by "The other one is buried." (from tennisplanet.me, following Madrid 2011 Djokovic d. Nadal 7-5 6-4)