Anyways, I searched for the videos and here's what I found. ( Links below )
To give you a rough idea of the imitations, here's dear Novak. :D

Video URLs :
Imitating lotsa players like Andy Roddick, Rafael Nadal (of course), Maria Sharapova and Roger Federer. For Roger Federer all he does is look graceful-and like he's World Number 1. ;) He also does the racket on palmthing Fed likes to do to get fans to clap for him.
http://www.youtube.com/watch?v=QDQHRpNL8d0
Imitating Sharapova and Rafa during the US Open. The whole crowd was roaring with laughter.
http://www.youtube.com/watch?v=lsMJB81HfFU
Imitating Roddick, Hewitt, Nadal, Sharapova, Sampras and one last one during his practice at Queens. Can't remember his name. But it's damn funny.
http://www.youtube.com/watch?v=VM6RUfeGLIE
Novak is really awesome at this. He even has the facial expressions down. :) To me, the Rafa imitation is DA BOMB. The BEST, by far. Talented, Djokovic is.. Adorable too. What a World Number 4.
He was actually asked about his imitations of other players, and when asked to do the Federer one, he went, " Roger is too perfect. "
http://www.youtube.com/watch?v=Vu7OKS9fU14
Hahahah! I love this boy.