Stefanos Tsitsipas fumed at his opponent utilising an underarm serve during the Greek's second-round defeat at the US Open on Thursday. 

Tsitsipas, 27, lost to German number two, Daniel Altmaier, 26, in a heated five-set clash at Flushing Meadows that finished 7-6, 1-6, 4-6, 6-3, 7-5.

Following the match, Tsitsipas poured fuel on the fire of what was a feisty encounter when he called Altmaier out for serving underarm, remarking: 'Next time, no underarm serve, okay?'.

In response, Altmaier told Sky Sports: 'Tomorrow Stefanos will regret his words. In football, fists fly or people get into arguments. 

'In tennis, you say something that might not be appropriate. That's part of the sport. A lot of money and self-confidence are at stake. 

'When you're playing tennis until midnight, you can sometimes say something you don't really mean. Or maybe he does. I respected that.'

Tsitsipas's coaching team were also warned for coaching him during the match, after his father, Apostolos, was seen shouting instructions to him.  

That marked the first time a warning has been given to a player on court since the International Tennis Federation (ITF) permitted it in matches earlier this year. 

ITF's Rule 30 allows 'brief, discreet verbal advice or hand signals from a coach, but only during breaks or permitted times, not during a point'. 

Altmaier added: 'These are the moments I play tennis for. It's hard to describe how I feel. There's a huge sense of relief. 

'They were two tough matches that gave me a lot of experience. That's the most important thing,' said the world number 56.

'It was more of a battle against myself. I always tried to keep believing in myself. 

'My dream is to win Grand Slam tournaments. I have to stay calm and perform at my best for two weeks.'

His reward is a third-round match against Australia's Alex de Minaur, which will take place on Saturday.