New US Open 'disrespect' row: Fired-up seed Stefanos Tsitsipas gives his underdog opponent an earful over his behaviour after he knocked him out

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' 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.