Fan kicked out of US Open match for saying "the most famous Hitler phrase," according to German Alexander Zverev

NEW YORK—Late-night matches at the US Open are known for incredible displays of stamina on the court, and equally incredible responses from the crowd seated close by. Sometimes, however, a line can be crossed between fan and player, and that was the case on Monday night/Tuesday morning.

At 2-2 in the fourth set of Alexander Zverev's fourth-round marathon with Jannik Sinner, Zverev stopped play to tell chair umpire James Keothavong that a fan in the lower bowl had "just said the most famous Hitler phrase there is."

"He just said the most famous Hitler phrase there is in this world," Zverev—a German—repeated to Keothavong, "It's not acceptable. It's unbelievable."

Upon hearing this, Keothavong immediately turned around to the crowd and demanded multiple times to know, "Who said that?" while pointing his finger.

When the answer wasn't immediately made clear, fans seated nearby—and, according to Kenny Ducey of @tennisbets, ESPN's Brad Gilbert, helped point out the fan to security.

The fan left without resistance, but an unmistakable stain had been left on the evening.

"A disparaging remark was directed toward Alexander Zverev," said USTA spokesman Chris Widmaier. "The fan was identified and escorted from the stadium."

Zverev clarified what he heard after the match, a 6-4, 3-6, 6-2, 4-6, 6-3 victory that concluded at 1:30 a.m. and lasted nearly five hours.

"He started singing the anthem of Hitler that was back in the day," Zverev explained. "It was 'Deutschland über alles' and it was a bit too much.

"I think he was getting involved in the match for a long time, though. I don't mind it. I love when fans are loud. I love when fans are emotional. But I think me being German and not really proud of that history, it's not really a great thing to do, and I think him sitting in one of the front rows, I think a lot of people heard it. So if I just don't react, I think it's bad from my side."