Djokovic is clearly the best player on the tour. The rankings don't lie.
But I'm not sure I agree with Russell that he's on such a different level that it's not possible to beat him. Within the last 12 months, he's lost to Nadal on 3 occasions (MC, Rome and RG), Murray on 3 occasions (Dubai, Olympics and USO) and Federer at least once (Wimbledon) and had some really close matches with them even when he won (AO final against Nadal, AO semi against Murray, Shanghai final against Murray).
He also had a narrow escape against Wawrinka at the recent AO. Wawrinka had chances to win that match.
It's certainly difficult to beat Djokovic. But it is by no means impossible.
"I have to apologise because I clearly pushed Roger so hard on Thursday!" (Andy Murray on why Federer couldn't play the WTF final).