I think Djokovic's 2011 season alone marks him out as an all-time great. If he had never hit another tennis ball after that, it should be sufficient IMO.
After all, what tennis player, who is not thought of as an all-time great, has ever achieved a season remotely like that?
"I have to apologise because I clearly pushed Roger so hard on Thursday!" (Andy Murray on why Federer couldn't play the WTF final).