World number one Amelie Mauresmo lost 6-4, 7-5 to defending champion Lindsay Davenport in the New Haven quarter-finals on Thursday.
Second seed Justine Henin-Hardenne eased into the last four but Mauresmo was outplayed by Davenport in a repeat of last year's final.
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 is 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.
"It's always disappointing to feel that you need a little bit more, more matches and everything. But it is much more positive than yesterday. Now I'm really looking forward for next week."
OTHER SEMI-FINAL
The other semi-final will be a repeat of this year's French Open final with Roland Garros winner Justine Henin-Hardenne taking on Russian Svetlana Kuznetsova.
Henin-Hardenne eased past Mara Santangelo 6-2, 6-3 to reach the last four and has beaten Kuznetsova in 11 of their 12 matches.
"Even if I have a good record against her, it doesn't mean anything right now because tomorrow is going to be other conditions," Henin-Hardenne said.
Former US Open Kuznetsova put out fellow Russian Elena Dementieva 6-3, 3-6, 6-2 to reach her ninth semi-final of 2006, and Stosur beat Marion Bartoli of France 7-6, 6-0.
In the men's event, second seed Nikolay Davydenko moved a step closer to his third title of the year by beating eighth seed Olivier Rochus of Belgium 6-4, 6-2 to reach the semi-finals.
The world number seven recovered from an early break down to take his place in the last four, where he will play Swede Robin Soederling.
"I never feel pressure to win because normally really I don't care about the future, winning a tournament or not," Davydenko said. "I just think about the next match."
Tenth seed Agustin Calleri of Argentina defeated Chilean Nicolas Massu, the 14th seed, 6-3, 3-6, 6-4 to set up a semi-final against Belgian Xavier Malisse who beat Juan Ignacio Chela of Argentina 4-6, 6-4, 6-2.
Soederling followed up his win over third seed Marcos Baghdatis with a 6-3, 6-2 win over Austrian Juergen Melzer to reach his third semi-final of the season.