Arthur Fils and Alexander Zverev clashed during a tense and dramatic Hamburg European Open final on Sunday.
World No 4 Zverev - who was born in the German city - was facing off against the 20-year-old Frenchman hoping to make it back-to-back titles on home soil.
Fils won the first set, before Zverev fought back, but the flashpoint came in the deciding set.
With the score at 5-5 and Fils a break point down, he unleashed a controversial underarm serve, which infuriated Zverev.
The serve was out but Fils was booed by the crowd and eventually won the game, which did little to abate Zverev's anger.
At the changeover, Fils confronted Zverev, with the German allegedly saying 'no respect' towards his counterpart, and the umpire was forced to step in to separate the two players.
The jeers towards Fils restarted, with the Frenchman simply pointing to his ear, before shrugging and shaking his head, something that riled them up further.
Fils ultimately went onto win the tiebreak to lift his first ATP 500 level title by virtue of a 6-3 3-6 7-6 (7-1) victory, yet the drama didn't subside there.
After Fils' winning point, the pair exchanged an extremely frosty handshake at the net, where Zverev barely made eye contact with his opponent, before he muttered something to himself and began to argue with the umpire.
Things did seem to eventually calm down, as Zverev sprayed Fils with champagne and poured the drink down his opponent's mouth.
They exchanged a smile as the trophy was presented to the Frenchman, who will be one of the main local hopes at the Paris Olympics.
Fils defended his position and actions when speaking post-match.
'I think that I did everything to win this match,' he said. 'I got cramps at 5-5 in the third.
'I'm cramping, trying underarm serves because I cannot serve.'
However, it is not the first time in recent times that the slightly hot-headed and abrasive Zverev has attracted attention.
Earlier this month, the German had an angry exchange with Taylor Fritz and clashed with the American's girlfriend, Morgan Riddle, after losing a five-set epic in the fourth round at Wimbledon.
Zverev - who had been two sets up before losing - accused members of Fritz's box of going 'over the top', while Riddle appeared to take aim at the world No 4, who has faced allegations of abuse, both during and after the match. Zverev denies any wrongdoing.
The German said: 'His team is extremely respectful. I think his coach, his physio, also his second coach, they're extremely respectful.
'I think there's some other people that maybe are in the box that are not maybe from the tennis world, that are not maybe from particularly watching every single match. They were a bit over the top.'
Meanwhile, Zverev, who was last year accused in court documents of physically abusing a woman in 2020, agreed to a settlement in the case last month. He agreed to pay Brenda Patea, the mother of his child, $217,820 (£168,679), but was found not guilty of any wrongdoing (Patea publicly identified herself as the victim in the case).
Last October, the Berlin-Tiergarten court issued a penalty order and a fine of around $488,000 (£377,907) against Zverev, who has denied Patea's claims of abuse. He called the order 'complete bulls***' a month later.
Patea told German newspaper Suddeutsche Zeitung that Zverev choked her during an argument. Zverev has also been accused of domestic abuse by Olga Sharypova, a fellow tennis player and another ex-girlfriend of his.
Sharypova made the allegations - also denied by Zverev - in 2020, accusing him of punching her in the face and smothering her with a pillow.
The ATP investigated the claims (via an outside firm) for 15 months but ultimately did not discipline Zverev on the basis of insufficient evidence.
Following his settlement in June, Zverev said: 'I told you so from the start, I told everybody. I'm happy that it's over. Yeah, nothing else more to say.'
