Andy Murray gave an unconvincing account of his US Open title credentials as he struggled to victory over Feliciano Lopez in round three.
Murray edged the first two sets and was twice a break up in the third, only to gift Lopez a route back just as the Spaniard looked a beaten man.
He laboured through 7-6 (7-5) 7-6 (7-5) 4-6 7-6 (7-4) in three hours 53 minutes on a baking Louis Armstrong Stadium.
The British number one
He is aiming to win a maiden Grand Slam title, but is likely to need a significant improvement for that to happen.
"I was struggling a bit physically and that made it challenging," said the world number four, seeded third in the absence of Rafael Nadal.
"I was up two sets and a break and should have done better there, but I played well in the tie-breaks and that was the difference.
"He played some really good stuff and served well when he was down. He was able to dictate a lot, win free points and play well at the net.
"It's the last major event of the year and you want to push through anything physically. I will be out here in the next match giving 100%."
Murray was forced to save five break points in the first set before taking it on a tie-break thanks to a couple of Lopez mistakes.
The Scot started the second with an early break of serve, but lost his focus with four errors in game six.
When Lopez held for 4-3 he had captured 12 points in a row, not to mention the initiative and crowd support.
Yet a dispute with the chair umpire at 5-4 and deuce seemed to distract the 30th seed, who again struggled in the tie-break.
Murray was now reading the Lopez serve and damaging his resolve with some relentless defence and incisive counter-punching.
An untimely Lopez double-fault put Murray 2-0 up in the third set, but he immediately relinquished his advantage with a forehand long.
Having moved 4-2 ahead courtesy of another double-fault, Murray again dropped serve and cut a forlorn, exhausted figure.
His usual movement and shotmaking was non-existent and Lopez leapt with delight as he broke once more and served out the set - the first he had taken off the Briton in three Grand Slam meetings.
Murray stuck with a rejuvenated Lopez throughout the fourth set and held firm for 6-5 in a game that featured an incredible 23-shot rally.
In the third tie-break of the match, Murray registered match point with a superb crosscourt backhand pass and was able to celebrate when his opponent sliced into the net.