Alexander Zverev's late-night US Open round of 16 match against Jannik Sinner was marred by an alleged racial slur made against the German No 12 seed.
Zverev and Sinner were embroiled in an enthralling fourth-round tie in the early hours of the morning in New York, when the former made a complaint to the umpire about a comment he heard in the seconds before a point.
Microphones courtside heard Zverev - born in Hamburg - telling the British umpire James Keothavong that a fan 'said the most famous Hitler phrase to me', before adding: 'It's unacceptable'.
In response, the umpire turned around in his chair to directly address the most expensive courtside seats, asking the culprit to identify himself.
When nobody owned up, Keothavong made an announcement to the whole crowd to respect the players, while security was dispatched into the area to try and remedy the unsavory situation.
Alexander Zverev was embroiled in a brilliant match in New York, but it was marred by an off-court comment he heard from a fan in New York
Jannik Sinner was Zverev's opponent for the feisty match, which went on late into the night
Play continued, before cameras then cut back to the stands a few minutes later to show a middle-aged male being spoken to by security and then led out of the arena - much to the delight of the fans around him.
It is unclear exactly what was allegedly said by the man, or if the man pictured by ESPN cameras was the fan alleged to have made the slur.
It is not the first crowd incident at this year's US Open, with Daniil Medvedev openly criticizing the supporters' noise after one of his matches.
'Thanks to all those who didn't yell between my first and second serve,' he said to the crowd after beating Sebastian Baez.
'However, there was a spectator, maybe he has a girlfriend or a wife who I don't know how she will sleep… He was particularly excited and will spend the night saying Vamos, vamos, vamos, without ever stopping. I'm sorry for him.'
The crowd in New York have been rowdy throughout the tournament. There is nothing to suggest the men pictured above have been involved in any wrongdoing.
Fans watch Carlos Alcaraz's match against Matteo Arnaldi on Monday afternoon
Elsewhere, other fans have commented on the 'feral' atmosphere being offered up - especially on the Labor Day weekend in the Big Apple.
In the match itself on Monday night, Zverev won the first set against his Italian opponent Sinner, who is ranked sixth and headed into the fourth round match as the favorite.
Sinner fought back to level at one-set-all, before Zverev made the most of his opponent's injury struggles to take the third set.
When all looked lost for Sinner with 1am approaching in the Big Apple, though, he won the fourth set to force a decider.
