Olga Govortsova of Belarus booked her place in the quarter-finals of the Bank of the West Classic by ousting second seed Samantha Stosur.
The world No 83 became the first to reach the last eight with a convincing 6-2 6-4 victory over the Australian former US Open champion at Stanford.
It is the first time Govortsova has reach this stage of a tournament this year and was helped along the way by 10 double faults from her opponent.
"It is inexcusable to get that many in one match," admitted Stosur. "She was playing quite well and I wasn't able to combat what she was able to do.
"Things were slowly turning around, and I had a break to go 5-5. I gave myself a chance to get back into the match but I had to win that last game."
Govortsova will play the winner of Sorana Cirstea or Coco Vandeweghe in the last eight after the latter won her first round match, beating Monica Niculescu of Romania 6-0 6-3.
Sixth seeded American Varvara Lepchenko of America beat Portugal's Michelle Larcher de Brito 6-2 6-4 to set up a second round match with Tamira Paszek, the Austrian beating Alla Kudryavtseva 6-1 6-7 3-0 when the Russian was forced to quit in the decider.
Urszula Radwanska, seeded seven, thrashed American Christina McHale 6-1 6-3 and will next play Daniela Hantuchova, the Bulgarian out-lasting Belgium's Yanina Wickmayer 6-2 4-6 6-0.