Novak Djokovic of Serbia reacts after a point against Alexander Zverev of Germany on day 12 of the 2021 U.S. Open tennis tournament at USTA Billie Jean King National Tennis Center.? Robert Deutsch-USA TODAY Sports

Novak Djokovic seemed to be irked after having his serve broken by Sascha Zverev in their semifinal match at the US Open on Friday night.

Djokovic lost the first set 4-6 but rallied to win the next two. The two men were on serve to begin the fourth set, but Zverev got an early break to go up 2-1.

Djokovic was upset about being broken and reacted toward his box, according to ESPN’s announcers. Chris Fowler and John McEnroe said Djokovic looked at his box and gestured to it with his hands. The announcers added that Djokovic spent the entire side change staring down his box.

It’s unclear what exactly what was going on with Djokovic.

The winner of their match will face Daniil Medvedev in the finals.

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