Wow, 21 is a lot... Where did you find that stat?I think, a crucial pount was also the setpoint in the third set. Fed is a frontrunner, and he would have a substantial lead in the match for the very first time. So, depite his better overall stats, in reality, given the tennis scoring, he was always a bit behind, with the exception of this 8-7 lead in the fifth. Against Nadal he was always in front, despite losing 1-6 in the second. Now, tennis is game of patterns, and it is not new, that Federer has problems to close out a vital match. He lost 21 matches having had mps on his own. It happened against Djokovic before at USO, it happened against Delpo and Anderson last year, against Thiem this year.
On the other hand, Djokovic was far from his best, his serve and return well below par. Despite his mediocre performance he came through on guts and resilience. This should be frightening for his opponents. What will happen on his beloved hard courts, when his is really on form? As dangermen for him i only see hard hitters like Khashanov and Medvedev, or Thiem, who is more a grinder, but has a big forehand and could outlast him.