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?
Andy Murray: "In front of the cameras I'm a little bit shy. When the cameras are off I'm a different person."