Belgian Yanina Wickmayer pounced on nine double-faults from ex-champion Marion Bartoli to knock the French second seed out in the quarter-finals of the WTA Stanford Classic 6-3, 6-2 on Friday.
The fifth-seeded Wickmayer will next play American Coco Vandeweghe, who reached the first WTA-level semi-final of her career with a defeat of Poland's Urszula Radwanska 6-4, 6-4.
Bartoli, the 2009 champion and three-time Stanford finalist, was plagued by errors during her defeat in 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, who was beaten in the 2011 final by Serena Williams, saved her first match point but then Wickmayer followed with a winner Bartoli couldn't get to.
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 ten 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 the Wickmayer, who was runner-up in 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 last month 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."
Top-seed Williams takes the court later in the day and is playing the week after her fifth Wimbledon title.