Heather Watson was unable to continue Britain's winning run at the US Open despite a great start against 21st seed Simona Halep.
It looked like the 21-year-old was going to join Dan Evans and Laura Robson in the second round in New York when she won the first set on Court 11.
But a costly slip-up in the ninth game of the second set turned the match and Watson fell away in the third to lose 4-6 6-4 6-2.
Although Halep is not one of the big names of women's tennis, she has been arguably the form player of the tour over the last three months.
The Romanian has won four of her last seven tournaments - only Serena Williams has won more during the season - with her latest victory coming in New Haven on Saturday.
Watson is still looking to rediscover her best form after suffering from glandular fever earlier in the year and has slipped to 76th in the rankings from a high of 39 in February.
But her fitness certainly appeared to be back and she made a terrific start, saving a break point in the first game before blasting a series of returns that Halep could not cope with.
Watson has been working on playing a more aggressive game with her new coach Jeremy Bates and she certainly was not holding back.
Halep fought back to 2-2 but Watson was playing especially well on the big points, saving break points in the fifth game and then breaking again to lead 5-3.
Back came Halep once more but Watson brought up a set point on her opponent's serve and took it when the Romanian netted a forehand.
Halep was becoming frustrated and flung her racket to the court when Watson held on to lead 2-1 in the second set.
The 21st seed did forge ahead for 3-2 but Watson broke back immediately with a drilled forehand winner down the line.
The Guernsey player was in the ascendancy and should have held for 5-4 but from 40-0 she lost five points in a row, double-faulting on the last one.
It was a bitter blow, and Halep capitalised by clinching the set.
The momentum had swung and the Romanian took control of the decider by winning the first three games.
Watson kept fighting but she could not put enough pressure on Halep, who sealed victory after exactly two hours when the British number two sent a backhand long.
A tally of 60 unforced errors rather told their own story for Watson, who also served eight double faults.