Teenager Andy Murray made an agonising exit at the US Open, suffering a 6-2 7-6 (7-2) 2-6 6-7 (4-7) 6-0 defeat to Frenchman Arnaud Clement.
The Scot looked poised to complete an incredible comeback when he recovered from two sets down to take the match into a deciding fifth set.
But as at Queen's and Wimbledon earlier this year, the 18-year-old was struck by cramp and ran out of steam.
Murray saved three match points at 0-5 but double-faulted on the fourth.
The British number three later insisted it was mental rather than physical fatigue that had contributed to his defeat.
"It's not easy coming back from two sets to love down," he said.
"In the fifth set when I got broken I ran out of gas - I've been working very hard over the last nine or 10 weeks.
"I just lost it after I got broken. I had nothing left, but it was more mental than anything."
MURRAY MATCH STATS
Double faults: 14
Unforced errors: 64
Break points won: 3 of 9
Murray took to the court knowing victory would see him break into the top 100 for the first time.
However, the teenager seemed overawed by the occasion and, as well as 21 errors, threw in six double faults as he lost the first two sets.
Clement, a veteran of eight US Opens, was playing solidly and showing glimpses of the form which took him to the Australian Open final in 2001.
But Murray, making his debut at Flushing Meadows, won yet more admirers with a gritty fightback.
He improved his serve and began to find the lines with his groundstrokes, breaking twice to take the third set 6-2.
The Scot, who came through in five sets against Andrei Pavel in the first round, then looked to have dealt Clement a crushing blow when he edged a tight fourth.
Murray missed three set points at 5-4, then saved break points against him in the next game as a dramatic set went to a tie-break.
With the crowd firmly in his corner, Murray raced into a 5-1 lead and after sealing it 7-4, the teenager looked favourite to clinch the match.
But there was another twist to come.
Murray, who recently hit out at those who questioned his fitness after Wimbledon, called for the trainer at 0-1.
He immediately received treatment for cramp in his right thigh but on his return, his serve had lost all penetration and his movement was clearly restricted.
The 18-year-old staged a brief rally in saving match points at 0-5 but he could not lift himself for another unlikely recovery.