Andy Murray suffered a disappointing 6-3 6-3 2-6 7-5 loss to Hyung-Taik Lee in the third round of the US Open.
Murray said he was disappointed by his lack of consistency
The British number one, who needed five sets to get through his previous match, was sluggish throughout.
However, his Korean opponent deserves credit for a solid display lit up by occasional flashes of brilliance.
Murray hit form in the third set and then fought back from 5-2 to 5-5 in the fourth but the 20-year-old's serve let him down again and Lee came through.
"It maybe took me five or six games to get going," said Murray.
"After that I was OK but maybe I gave myself a little bit too much to do.
"If I had got one of those chances in the first set and maybe got myself a set ahead, I might have loosened up and started to play better."
Lee, 31, is through to the fourth round of a Grand Slam for only the second time in his career where he will face Russian fourth seed Nikolay Davydenko.
It was a brave performance from Lee, who looked to have blown his chance of upsetting the 19th seed when he let slip a commanding lead in the fourth set.
He had taken complete control of the match by breaking once in the first set and twice in the second.
Murray, who came into the tournament desperately short of match practice following a wrist injury, was struggling on serve.
His return, which he considers one of his strengths, was also misfiring.
His first-serve percentage was a miserable 43% in the first set before creeping up to 55% in the second and in that time, he managed to earn only one break point.
But the British number one, who came from two sets to one down to beat Jonas Bjorkman in the previous round, did at least put up a tremendous fight before going down.
The momentum swung in his favour when he broke serve for the first time in the match at the start of the third set and he promptly repeated the trick to take a 4-1 lead before confidently serving out.
Lee appeared to be on the rack when Murray broke serve immediately in the fourth but crucially, the 19th seed threw in another careless service game to allow the Korean to break straight back.
Murray, who struggled to contain his frustration throughout, hurled his racquet across the court, prompting boos from a large Korean support camp on Grandstand court.
Despite needing treatment on a back injury, Lee broke serve to take a 4-2 lead and then held to love to make it 5-2, as Murray looked a spent force.
But the former US Open junior champion went down fighting.
The Korean had a match point at 5-3 but put his attempted pass too close to Murray and the Scot punched away an instinctive volley.
After taking advantage of a nervy game from Lee to break back, Murray held serve to love to make it 5-5.
But at 6-5 down, the Briton's serving woes came back to haunt him and a lazy backhand into the net gave Lee a match point, which the Korean took with a brilliant forehand down the line.