You totally are getting mixed up between mental toughness of an inferior athlete and a fantastic athlete who is also a great tennis player.
You are the one who does not know this game. Djokovic is YOUNGER, and got more control over his game mentally while his major compe ors, Nadal and Federer, are on their way out. Djokovic was the guy that SHOULD beat these guys because he IS a better athlete, even better than Federer during his prime. Djokovic moves better imo. I believe Federer was the pinnacle of the beautifully efficient Tennis stroke. NOW Djokovic is beating these guys (so are a lot of other players) while totally dropping the ball on what would have been one of the greatest feats in Tennis.
You dont get it.
Who surrounds you makes you great. Djokovic has gotten past the point of these guys being able to beat him. They are just not good enough, they are way past prime. So a German kid slaughters him in Straight SETS and he gives up? Thats not the model.