It has been the shot that has been viewed as Andy Murray's weakness throughout his career and now it might just be the flaw that ends his story.
Murray was trying to reach the last 16 at a slam for the first time since his hip problems began in 2017 but he fell to a 6-4 6-4 6-7 (1) 6-3 defeat against Matteo Berrettini on Arthur Ashe Stadium.
As the scoreline confirms, Murray was highly competitive against the 2021 Wimbledon runner-up, yet it was his serve that let him down again when he needed it most.
The Scot has done a remarkable job of covering up serving problems that have undermined his efforts throughout his remarkable career, with his sublime all-court talents and fighting spirit ensuing he will head into retirement with his legacy as the greatest British sportsman of all-time secure.
If Murray's career had fallen in an era that didn't include Roger Federer, Rafael Nadal and Novak Djokovic, the Scot would have undoubtedly have more Grand Slam titles than the three he won at the US Open in 2012 and at Wimbledon in 2013 and 2106.
Even in his finest hours Murray's serving problems have never been far away from the surface, with his booming first serve not backed up have a second serve to match.
All opponents have targeted Murray's second second down the years and the same story was played out in his latest match, with Berrettini benefitting from his opponent's first serve breaking down alarmingly in the first two sets.
The final statistics confirmed Murray got 53 per-cent of his first serves in, but that number was boosted by his move to take pace on his first serve in the third set as his exasperation over the missing delivery was evident for all to see.
Murray threw double faults in at inopportune moments and was constantly fighting to hold on to his service games until he was decisively broken at 3-4 down in the fourth set, with Berrettini taking advantage to hammer nails into his rival's US Open hopes.
"I served pretty poorly for a large part of the match, which hurt me a lot. I just couldn't find any rhythm on serve," conceded Murray.
"I hung in really well in the third set and you could see from the stats that, when I was getting into rallies, I was getting comfortably the better of those exchanges when I got past the first few shots.
"He served extremely well, got loads of free points on his serve. I didn't. That was the difference."
There have been many more crushing defeats than this in the latter part of Murray's career and for that reason, there is no reason why he should consider his future at the age of 35.
Yet Murray was defiant in his post match interview as he insisted retirement was not on his mind.
"I want to push and see how far I can go. That will be motivational for me and interesting to see," he said.
"Lots of people told me I wouldn't be able to play again, and lots of people told me I'd be able to hit tennis balls but not compete professionally again.
"That was nonsense, and I want to see how close I can get back to the top of the game. Yeah, that's what the motivation is."
Murray's form in the last few weeks suggests he can get back into the world's top 30 imminently and he can compete against the game's finest players, but this defiant competition always used to come out on top in matches when the battle lines were drawn and the competitive juices began to flow against opponents like Berrettini.
All too often in the last couple of years, these tight matches have gone against Murray and unless he can solve his serving problems, that flaw will continue to haunt him.