A fan was ejected from a U.S. Open tennis match early Tuesday morning after German player Alexander Zverev complained the man used language from Adolf Hitler’s Nazi regime.

Zverev, the No. 12 seed, was serving at 2-2 in the fourth set of his match against No. 6 Jannik Sinner when he suddenly went to chair umpire James Keothavong and pointed toward the fan, who was sitting in a section behind the umpire.

“He just said the most famous Hitler phrase there is in this world,” Zverev told Keothavong. “It’s not acceptable.”

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“He started singing the anthem of Hitler that was back in the day. It was ‘Deutschland über alles’ and it was a bit too much,” Zverev said.