Yanina Wickmayer pounced on nine double-faults from Marion Bartoli to knock the French second seed out of the quarter-finals of the WTA hardcourt tournament in Stanford on Friday.
Belgium's Wickmayer, seeded fifth, ousted Bartoli 6-3, 6-2 to advance to a semi-final clash with American Coco Vandeweghe.
Vandeweghe reached the first WTA-level semi-final of her career with a 6-4, 6-4 victory over Poland's Urszula Radwanska.
Top seed and defending champion Serena Williams, playing the week after capturing her fifth Wimbledon title, was due on court later against South African Chanelle Scheepers.
Waiting for the winner of that contest will be Sorana Cirstea after the Romanian roared back to upset Slovak third seed Dominika Cibulkova 6-7 (5/7), 6-2, 6-0.
Cirstea coped with a bevy of incorrect line calls, but won six Hawk-eye challenges confirmed by the electronic line calling system.
"After that many, I didn't trust anyone anymore," she said. "I felt like someone was playing a joke on me. There were too many mistakes, I've never experienced this.
"It was just hilarious. It was everyone, not just one linesperson. The chair umpire didn't over-rule anything. I'm thankful we have this (Hawk-eye) system. It was important to stay in the match mentally."
The Romania also helped her own cause with 15 aces and won the last 10 games on the way to victory.
Bartoli, the 2009 champion and runner-up to Williams in 2011, was plagued by errors in a match lasting just under 90 minutes.
She double-faulted twice in what proved to be the final game when she lost serve for the fourth time.
Bartoli saved the first match point she faced, but Wickmayer followed with a winner Bartoli couldn't reach.
It was world number 37 Wickmayer's first win over the 2007 Wimbledon finalist and world number 10 in four meetings.
Bartoli considers the Stanford University campus one of her favourite venues, having played the event 10 times. Coming into the contest with Wickmayer, she had won 20 of her last 29 matches played in California.
Wickmayer, now into her fourth semi-final of 2012, kept Bartoli well off the top of her game.
"I was focused even when I was leading," said Wickmayer, a finalist at two tournaments this season. "I played some of my best tennis today.
"I had tough three-set wins this week and I think they helped me today. I was happy I could keep my level up against Marion."
Vandeweghe followed up on her second-round win over former number one Jelena Jankovic by beating Radwanska, who is the younger sister of world number two and
Wimbledon finalist Agnieszka Radwanska.
The 21-year-old American, who is coached by former ATP player Jan-Michael Gambill, got into the event as a lucky loser.
She played a lower-level grass tournament final in June in Nottingham after going through qualifying rounds.
"Mentally that helps, you realise you can win these matches," said Vandeweghe, who is working on a return to the top 100 after suffering an injury following the Australian Open.
"I'm playing match by match and putting in the hard work. If you do that it can translate on court. I had confidence today after beating Jankovic."