Who is better player Muray or Roddick?? He was second when Fed won everything!! Had 13-1 or even 13-0 against him!! Federer won all tha because after Sampras retired there was no competition until Nadal came..
When I say that Djokovic does not have the rivals that Federer and Nadal had, I am actually talking about Federer and Nadal themselfs.
Federer dominated tennis until 2005 basically alone (he got 6 GS till there) because Roddick was good but never like them (Sampras was already out of competition).
Andy Murray is better than Roddick but is not that good to put him in another level like Federer Nadal or Djokovic.
I agree that the head to head and the four Gran Slam finals won by Roger against Roddick talk for itself and put the American in a very bad position, maybe if Roddick had won a couple of more matches against Federer it should seem to be more equal.
What I want to say is that nowdays Djokovic does not have a Nadal or a Federer (even Roger is still number 2 is far away fisically), when Federer was at his very best he had a monster called Rafael Nadal and when Nadal was at his very best has a the other monster called Roger Federer and then, Djokovic specially in 2011.
I think Djokovic is a great player, and he is one of the best ever, but the sport is right now missing someone who challenged him more, and that guy is probably Rafa, but he is struggling quiet a lot these days and Federer but at his age is very difficult!