Back in early 08 it looked possible. He was challenging for number two. Nadal had only won slams on clay. He had just beaten Federer in a grand slam.
Now it doesn't look so hot. Nadal's dominance has spread to other slams. Federer rebounded from his Australian Open loss to take the U.S. and now the French. And of course Murray's risen through the rankings like a meteor.
But things change quickly in tennis. Djokovic is streaky. Right now he's having a bad streak. In a few months, or maybe a year or two, he might go on another good streak like in early 08.
I'm not counting him out.