World number one Amelie Mauresmo lost 6-4, 7-5 to defending champion Lindsay Davenport in the New Haven quarter-finals.
Second seed Justine Henin-Hardenne eased into the last four but Mauresmo was outplayed by Davenport in a repeat of last year's final on Thursday.
Having struggled through her opening match against Russian qualifier Galina Voskoboeva, Mauresmo must have feared the worst against Davenport having lost their past 10 meetings.
Although Davenport was playing only her fourth match since returning from five months out with a back injury, she never allowed Mauresmo to get a foothold in the match.
One break, in the third game, was enough for Davenport to take the first set.
Mauresmo, the Wimbledon and Australian Open champion, recovered an early break for 3-3 in the second set but the next five games went against the serve before Davenport held on to clinch victory.
Davenport will play Australian lucky loser Samantha Stosur in the semi-finals on Friday.
"To beat a player of her level so soon is extremely rewarding," Davenport told reporters.
"Whatever happens tomorrow happens tomorrow. It's still a huge success for me to be in the semi-finals and to have accomplished what I've accomplished.
"I know I need to take baby steps to come back. At the same token, you know, I'm happy that I'm playing well and feel like I want to keep on going."
Mauresmo, in her first event since Wimbledon after delaying her return to the Tour because of a shoulder problem, said she was just missing a bit of match practice.
"I think tonight I had some great moments but was not able to keep it up for the whole match, especially on my service games at the end of the match," she said.