A fan has been ejected from a US Open round-of-16 match 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 inside Arthur Ashe Stadium.

"He just said the most famous Hitler phrase there is in this world," Zverev told Keothavong.

"It's not acceptable."

Keothavong turned backward and asked the fan to identify himself, then asked fans to be respectful to both players. Then, during the changeover shortly after Zverev held serve, the fan was identified by spectators seated near him, and he was removed by security.

Zverev said after the match that he's had fans make derogatory comments before, but not involving Hitler.

"He started singing the anthem of Hitler that was back in the day ... and it was a bit too much," Zverev said.

"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."

Zverev went on to drop that set, when he began to struggle with the humid conditions after Sinner had been cramping badly in the third set. But Zverev recovered to win the fifth, wrapping up the match that lasted four hours 41 minutes and finished at about 1.40am on Tuesday. He will play defending US Open champion Carlos Alcaraz in the quarter-finals.

Zverev said it wasn't hard to move past the fan's remark.

"It's his loss, to be honest, to not witness the final two sets of that match," Zverev said.