Anna Chakvetadze saddled Venus Williams with her first loss San Diego Classic defeat since 1999 on Friday to set up an all-Russian semifinal with Maria Sharapova.
Chakvetadze, the third seed who arrived at Carlsbad seeking her third title in as many weeks, defeated Wimbledon champion Williams 6-7 (5/7), 7-6 (7/3), 6-2.
Chakvetadze struggled through a collapse in the first set, throwing away a 5-2 lead, missing on two set points and finally losing a tiebreaker.
But the 20-year-old hung tough, taking the second set in a decider as Williams stuffed an easy volley into the net after two hours on court.
The third set turned into a runaway for the Russian, who has now won a dozen straight matches. Williams was stunned after striking 79 unforced errors and missing on a match point.
"She saved her best for San Diego," Williams said. "I was not at my best level. I'm really tired and it showed. Lots of things went wrong in my game tonight."
Chakvetadze said that the crowd of 5,500 helped pull her through the marathon that lasted two and a half hours.
"I'll have to try and do my best tomorrow, but I don't know what my strategy against Maria will be. I'm tired now and will have to see how I feel on the day."
"I'll try to move her around and hope for the best," said the winner of July events in Cincinnati and Stanford.
By contrast, top seed Sharapova earned an efficient 6-2, 6-1 win over India's Sania Mirza to book her her place in the final four of the 1.3 million-event.
Sharapova has won her last two events against Chakvetadze, both played this season, leading the series 4-1.
Swiss 11th seed Patty Schnyder continued her roll, upsetting Russian fourth seed Nadia Petrova 6-4, 6-4 to reach only her second semifinal of the season. She had stalled seven times at the quarterfinal stage this year.
She will play ninth seed Elena Dementieva, who moved into her second semi of the season with a 6-2, 6-4 victory over fellow Russian Maria Kirilenko.
Schnyder followed up her three-set win over compatriot Martina Hingis in Thursday's third round with a comprehensive victory against Petrova.
"I made her run a lot and really got my game going well," said Schnyder, who played a third straight quarterfinal after Cincinnati and Stanford.
"I'm feeling great on the court, especially these past three days. Everything seems to fit well in my game. I just want to keep this up for the weekend."
Sharapova, playing through a lingering shoulder problem, was pleased with her performance.
"I had a good start and was able to play my game," said the world number two, one of five Russian to reach the quarterfinals during the final edition of this tournament.
"Sania is a solid player, so it's good to get some matches like I've won this week behind my back."
Sharapova lost serve only once, in the second set, but got it straight back on the way to victory in just 61 minutes.
"I've had three good matches, I much prefer it this way than running around for a long time in the hot sun to win," added Sharapova, who is nursing a sore shoulder. "I've got confidence, and I'm ready to step it up in my next match."