I think what we're forgetting is that it's one thing to be a professional tennis player. It's another thing to be in the spotlight for a bit. But none of that applies to Djokovic. What he's going through is way more than any of that.
when you're number one, and have been for a long time, and people start telling you that you could be better than Federer and unseat him as the GOAT, well, that kind of celebrity can be a bit much for someone. That kind of celebrity is something you don't train for. For most, it's a crap shoot. But for Djokovic, it's right in his face -- and he seems to not be able to handle it like Federer has.
He seems to be crumbling under the pressure and expectations of being the best. People no longer root for him to win -- they expect him to win. That kind of pressure isn't something he's clearly ready for, and it is taking its toll.
Plus, everyone's gunning for him. He's got so many points to lose and people have been chipping away at his ranking all year long.
It is now way more than just "tennis". Getting onto the court to play a match is really furthest from his mind right now, unfortunately. There was a time when that was the biggest, most important thing. But now, this most important thing to him is keeping it together -- emotionally.