First off. Yes, very frustrating.
That being said, come on. You know what he was doing, and if DelPo did that during a Djoker match, or Rafa (both do similar things), and I like both of those guys, you would have an issue.
It is right there, clear as day. We saw it last year at the USO until he played Wawa where it simply was not going to work.
Djoker plays mind games when he is struggling. You can argue whether or not it is bad sportsmanship, but it is right there in front of everyone. Announcers see it, fans see it, and other players see it.
Only Djoker fans don't.
But hey, it clearly works.
But to be fair, maybe he is not aware that he is doing it. Maybe it is just his mental self battling, and he usually battles through it. But it sure has become a trend.