Fourth seed Elena Dementieva upset top seed Justine Henin Hardenne 2-6, 7-5, 7-5 to reach the Pacific Life Open final on Friday.
Dementieva will face third seed Maria Sharapova, who beat Martina Hingis 6-3, 6-3, in the final.
On the men's side, American James Blake overwhelmed Igor Andreev of Russia 6-1, 6-4 in the quarter-finals.
Blake will face the winner of the match between number two Rafael Nadal and 25th seed Marcos Baghdatis of Cyprus.
Henin came into the match with a 7-1 winning record over Dementieva and cruised through the first set and a half before the wheels came off with the score at 2-5.
A notoriously slow starter Dementieva suddenly found her game, running down ball after ball and rarely making errors.
She broke Henin twice to get back to 5-5 and then wore her bewildered opponent down to take the set.
Henin had her right knee wrapped in between sets and regained momentum in the third set.
Dementieva held a match point with Henin serving at 4-5 but the Belgian came up with an overhead winner.
With Henin serving at 5-6, however, her legs simply gave out and she committed two backhand errors to lose the two-hour, 51-minute contest.
"I was dominating and then I lost my concentration," said Henin. "I helped her come back into the match. Maybe I thought the match was over too soon and she turned the match completely around. She's a great fighter."
STERLING PERFORMANCE
Sharapova gave a sterling performance against Hingis, wearing her opponent down in long rallies, attacking the Swiss player's second serves and bullying her from the forehand side.
Hingis tried to move the 18-year-old around the court but Sharapova, who was suffering with blisters, stayed in the points and put away any mid-court balls with aplomb.
"The score doesn't show how physically and mentally tough every point was," said the Russian, who fired 29 winners against only eight from Hingis.
"The quality of the match was very high level. Mentally, I told myself: 'It doesn't matter how many balls she gets, do everything possible to get one more ball back'."
Hingis was broken six times during the one hour 38 minute contest and only won 37 percent of her second serve points.
Sharapova is now 2-1 against the Swiss player, losing their first meeting at Tokyo in February before beating her in Dubai.
While she is known for her first pumps and self-motivating yells, Sharapova was particularly fired up against Hingis, especially after a fan called out in the second set, saying that she was tired.
"That kind of pumped me up a little," Sharapova said. "I hit two winners in a row. I looked back at him, and I'm like, 'Tired, my butt. So don't mess with a truck. You're going to become a pancake."
