Watch: Stefanos Tsitsipas and Daniel Altmaier engage in heated handshake over underarm serve at US Open

Stefanos Tsitsipas was slumped out of the US Open on Thursday in the second round but he didn't take it lightly after his opponent Daniel Altmaier decided to underarm serve to end the tie.

Tsitsipas lost to German number two Daniel Altmaier 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 called out Altmaier for serving underarm.

“Next time, don’t wonder why I hit you, OK? No, I’m just saying if you serve underarm ... if you serve underarm,” Tsitsipas told Altmaier.

But Altmaier then decidingly said that Tsitsipas would regret his words and that playing till midnight and a mix of that and a lot of money and pride being at stake likely accounted for him getting heated so he gets it in the end.

"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."

"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,' added 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."

Tsitsipas also saw his dad Apostolos receive a warning for coaching him during the match. This is the first time a warning has been given to a player on court since the ITF permitted coaching earlier this year with brief, discreet verbal advice or hand signals from a coach, but only during breaks or permitted times, not during a point being the ruling. The Greek went against that and was reprimanded. So overall for a player who is struggling to stay at the top table of men's tennis it wasn't the best way to end the contest.