Elena Baltacha was knocked out of the Texas Open quarter-finals 6-2 6-2 by France's Aravane Rezai, who is ranked 53 places below the British number one.
Baltacha had fought back from a set and break down to in the previous round.
But the world number 61 had little joy serving against Rezai, producing four double faults and also failing to win any of her three break points.
Wildcard Jamie Hampton awaits Baltacha in next week's US Open first round.
Baltacha, who was beaten in an hour and 24 minutes, is aiming to make the second round of a Grand Slam for the fifth time in a row at Flushing Meadows.
"Today was disappointing but Aravane is a very talented player and she was on today, while I was probably just a little bit off," said the Scot.
"Overall this has been a great week for me, though, and the ideal preparation for the US Open. It's given me loads of confidence because I've played well in tough, very hot conditions in Dallas."