The key is being unpredictable. That means being capable of hitting any shot in any direction, with any type of spin. This also applies to the serve. Federer is the master of that. Murray is getting there. More often than not, that sends Djokovic into this semi-conservative mode where he's just kind of hitting heavy topspins and underwhelming backhand slices, trying to win the positioning game. The Big 4 have weapons that can break Djokovic down at this stage.
Easier said than done though. All players, even Federer, have their tendencies. Once Djokovic locks in on those tendencies, he becomes considerably more offensive-minded, and unless he's struggling with his mechanics or physically hurt, it's usually all downhill for him at that point.
Then there's this God 2.0 mode he sometimes goes into when his back is to the wall, where nothing really matters except for Djokovic's own form. It's a depressing reality for most of the tour, but when Djokovic decides f--- it, and starts going on an all-out offensive blitz, the only thing that can stop Djokovic is Djokovic.