If he could just use that serve at will, why did he stop using it when it was a success at the USO? Also, Nadal is not beating Djokovic or Murray on hard I think. At the USO Fed can beat him too.
Partly due to shoulder issues, partly due to the fact that he doesn't like having a fast serve with lower percentage, because he doesn't lie to play such a game with shorter points, he gets out of rhythm. Someone posted that "Nadal doesn't want to serve big because it doesn't fit with his pusher game" was somewhat accurate.
If Nadal starts serving faster, he's in with a shot vs. anyone at USO 2013.