I only keep one eye on tennis stuff most of the year (and that get caught up in it again around Wimbledon), but it seems to me Djokovic and Murray are the two rising stars of men's tennis at the moment.
I might be proven wrong, but I have the feeling Djokovic will be consistently good, but never quite make it. Like, he'll make Grand Slam semis quite often (and maybe even the odd final), but I'm just not sure he has what it takes to win one.
Murray on the other hand I could see being all over the place. Sometimes going far, sometimes going out early...but I think he has the arsenal to actually win a Grand Slam at some point, even with the likes of Nadal and Federer around.