Andy Murray felt let down by his body as he wearily exited the US Open at the quarter-final stage in a four-set loss to Novak Djokovic.
It was past 1am in New York when the world number one finally finished off a 7-6 (7/1) 6-7 (1/7) 6-2 6-4 victory after three hours and 32 minutes to move forward to a semi-final against Japan's Kei Nishikori.
The contest was brutally physical, as matches between the pair always are, and Murray was clearly suffering during the fourth set.
His service speed dropped below 100mph and it was not a surprise when Djokovic pounced in the 10th game, clinching victory on his first match point when Murray netted a backhand.
The Scot said: "I'd say definitely physically he was fresher, but towards the end I tried to hang in as best I could in the fourth set.
"He appeared fresher than me. Whether he was or not I don't know - maybe he does a better job of hiding it than me. The pace of my serve slowed significantly towards the end of the third set."
The considerable consolation for Murray was that he played his best tennis since back surgery almost a year ago and looks close to the form that brought him two grand slam titles.
"I played well," said the eighth seed. "Especially the first couple of sets there was some good tennis. I played a poor tie-break in the first set definitely.
"I was down in the first set and I fought back. I was down in the second set and I fought back. So I fought hard. I played some good tennis. But it wasn't enough.
"Right now I'm obviously disappointed. It's extremely late. I'm tired. I don't feel particularly proud right now. But I think there was some good tennis. Hopefully I can build on that."
This was easily the most eagerly-awaited clash of the tournament so far and a match-up that has decided four grand slam finals - both men winning two each.
Murray had the advantage of having won their only previous meeting here, when he broke his grand slam duck in 2012, and their last grand slam match, last summer's career-defining Wimbledon win.
There was no doubt he was the underdog this time, though, with Djokovic back at world number one, the Wimbledon champion and through to the last eight without having dropped a set.
The New York crowd settled in for a long night as Djokovic began the contest at 9.43pm, and it quickly became clear service breaks would not be at a premium.
Having made the perfect start with a break of the Djokovic serve, Murray found himself 4-1 down before fighting back to level.
It was anyone's game heading into the tie-break but Murray dropped his first service point with a double fault and never recovered.
Murray had never beaten Djokovic from a set down and the world number one had never lost a match at Flushing Meadows from a set up, so the odds were firmly in his favour.
Djokovic's record in New York is seriously impressive, with the 27-year-old now through to an eighth consecutive semi-final and bidding to reach a fifth straight final.
Murray was twice a break down in the second set but doggedly kept clawing his way back, his flat cross-court forehand becoming the match's signature shot.
Djokovic saved a set point at 5-6 but this time it was the world number one who made a poor start to the tie-break and Murray won it easily.
The Scot looked at his best right at the start of the third set but he played a loose game to be broken for 3-1 and never managed to wrest the initiative back.
Video: Looking ahead to day eleven at the US Open
A tally of four break points taken from 16 created showed that Murray did not lack opportunities but could not make the most of them.
The eighth seed had spent an extra three and a half hours on court in getting to the last eight so it was perhaps not surprising he began to struggle first.
He received a heat pack from the trainer for his back but his serve was stiff and awkward.
Any concern over Murray's back is a source of worry because of the problems he went through that eventually prompted him to undergo surgery, but he reported no serious issue.
"I got stiff in my hips and my back towards the end of the third set," he said. "I don't know exactly why but I'm certainly not injured. I didn't hurt anything. It was just, I think, fatigue."
Murray arrived in New York declaring himself in the best shape since last year's Wimbledon following a training block in Miami.
Two days later he cramped dramatically against Robin Haase in his first-round match and was slightly disheartened that again he could not give his best late in the match.
Murray prides himself on being one of the fittest players on tour, and he said: "I was a little bit disappointed because I did train very hard.
"You do the work for these matches, so I would have liked to have felt a little bit better towards the end."