He seemed to get annoyed that Nadal kept returning the ball and it looked like he really wanted to smash the ball, but ended up messing it up completely. At least that's how it looked to me anyway.
Was very impressed with Nadal though. Djokovic looked all over him throughout the 2nd set and into the 3rd, but Nadal hung in there in the 3rd holding his serve, even though he was constantly on the back foot. Djokovic looked comfortable on the other hand in the 3rd, until he had to serve to stay in the match, where it has to be said he choked.