Britain's Laura Robson lost in straight sets to 30th seed Anabel Medina Garrigues in the second round of the US Open.
Robson, ranked 173 in the world, saw her New York campaign end as she went down 6-2 6-3 to the Spaniard.
The 17-year-old Londoner showed flashes of the form that saw her win three qualifying matches and another in the first-round proper on Monday.
But she was made to pay for 32 unforced errors against Mediana Garrigues.
Robson, competing in the main draw for the first time, had booked her spot in the second round when Ayumi Morita was unable to continue because of a shoulder injury in her opening match.
However, Robson had won the first set in that match and looked in confident form again as she used her big serve to good effect until the fifth game, when Medina Garrigues broke to take a 3-2 lead.
The Spaniard went on to see out the set comfortably, closing it out with the first ace of the match, but Robson raised her game early in the second set.
Leading 3-2, the Briton had two chances to get a vital break of serve but she could not convert and Medina Garrigues reeled off four straight games to seal victory and a third-round meeting with world number two Vera Zvonareva.
"I'm obviously really disappointed because it was quite clear I just made too many mistakes," said Robson, the youngest player in the women's top 200.
"I think I tried to go for too much. I felt like I had to get in there first. She played really tactically smart."
And Robson could reflect on a highly successful week in New York that included four victories and marked another significant step forward in the senior game.
"I'll take a lot of positives," she said. "It was my first time qualifying for a Grand Slam and I won a round. I'm still happy with the way I'm playing, I think today she just had more experience and that showed.
"I will always play aggressive tennis, I think that's the game that suits me. Just because I miss a few here and there doesn't mean I should go defensive all of a sudden.
"I had things I said I wanted to improve on after Wimbledon, one of them being consistency, and I think that's still the main thing I can work on, and just continue to get fitter and faster and start working on my tactics.
"Before I just went out there and played but now I think I've got enough options with my shots that I can do various different things."