Just before losing the third set of his U.S. Open semifinal match against Gael Monfils, Novak Djokovic ripped his shirt nearly in half before shedding it and opting for a new top.
Djokovic had taken the first two sets of the match, 6-3, 6-2, before dropping the third 3-6. While it’s normal to change shirts after a set, it’s not normal to rip them off. Perhaps the Serb’s shoulder was giving him issues (he has been dealing with discomfort all tournament), or perhaps he was a bit upset a his play.
Judging from that video, the shoulder is strong.