A fan was ejected from Arthur Ashe Stadium early Tuesday morning after German tennis player Alexander Zverev complained the man had used language from Adolf Hitler’s regime during his fourth-round match at the U.S. Open.

Zverev, the No. 12 seed, was serving in the fourth set against sixth-seeded Jannik Sinner of Italy when he stopped play. Zverev ventured toward chair umpire James Keothavong and pointed at the fan, who was sitting in a section behind him.

“He just said the most famous Hitler phrase there is in this world,” Zverev told Keothavong. “This is unacceptable. This is unbelievable.”

Keothavong turned around and asked the man to identify himself. He did not, and the chair umpire urged fans to remain respectful of both players.

Security personnel removed the fan after he was identified by others around him.

“A disparaging remark was directed towards Zverev,” a United States Tennis Association spokesperson said in a statement. “The fan was identified and escorted from the stadium.”

CNN reported that the fan uttered a phrase from the German national anthem, that since has been removed, that became a rallying cry under Hitler during World War II.

Zverev, 26, was able to refocus and record a 6-4, 3-6, 6-2, 4-6, 6-3 win in 4 hours, 41 minutes. He advanced to face defending U.S. Open champion Carlos Alcaraz of Spain in the quarterfinals.

–Field Level Media