Alexander Zverev's late-night US Open round of 16 match against Jannik Sinner was interrupted by a fan who yelled out a famous Hiterl quote 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.
'He just said the most famous Hitler phrase there is. It is unacceptable,' Hamburg-born Zverev told British umpire James Keothavong after hearing a slur from the crowd.
Following the game, Zverev told a reporter that the supposed fan had been singing the 'anthem of Hitler' - the banned first verse of the original German National anthem.
'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,' the Hamburg-native said.
'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,' he added.