Jannik Sinner apologised to the umpire after going well over his allotted 25 seconds on the serve clock.
Straight after, the Italian sent an absolute ripper down the court to leave Jack Draper stumbling to land the ace.
The serve clock has been utilised in ATP and WTA tour events in a bid to quicken up games.
It was first introduced in 2018 and before it became widespread at tournaments in 2020.
Jannik Sinner was guilty of going over his allocated serve time
US Open
But during the second set of their US Open semi-final, Sinner was guilty of going well over the allotted 25 seconds.
The world No. 1 had turned to use a towel and seemed oblivious to the clock timer, which had even stopped itself on two occasions to give Sinner the chance to return.
However, even with the small stoppages, he still went over the time by a wide margin.
Sinner did realise his error and before his serve, he raised an arm to the umpire and apologised.
A punishment is up to the umpire but players are usually given a warning before potentially being docked a point if found guilty again.
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But the umpire let Sinner off with his mistake without a warning - who then delivered a deadly ace before taking the second game in the second set.
Fans were not impressed watching the US Open, with one saying: "Sinner defo bend the rules on the shot clock."
Another wrote: "What happened to the shot clock and why did Sinner get to flout it?"
And a third said: "How’s Sinner got away with going over the clock there?"
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Jannik Sinner apologised for breaking the rule and was allowed to carry on
US Open
In defence of Sinner, the set had already dragged into its 22nd minute of non-stop play with neither players being allowed to sit down and have a break.
While the rules do say you cannot go over the 25 seconds allocated, the umpire is allowed to use some discretion considering the situation the game is in.
Both players were dripping in sweat and looked leggy after a first set that lasted just shy of an hour, eventually going Sinner's way 7-5.
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Jannik Sinner is No. 1 in the world
US Open
It was the first set that Draper had dropped at the US Open this year while he is already in unfamiliar territory.
The 22-year-old had never reached a quarter-final of a Grand Slam event - let alone a semi-final.
