Eighth-seeded German Andrea Petkovic held on for a three-set win over Venus Williams on Friday that scuppered a potential All-Williams Stanford semi-final.
World number one and top seed Serena Williams was in action later Friday, taking on Serbia's Ana Ivanovic in the quarter-finals of the hardcourt WTA tournament.
The winner will face Petkovic, who stormed to a 6-1, 2-0 lead over Venus Williams then survived the American's rally to win 6-2, 3-6, 7-5.
Williams, who had beaten fourth-seeded Victoria Azarenka on Wednesday night, won six of the last seven games of the second set to force a decisive third set against Petkovic.
Petkovic regrouped to gain an early break in the third, only for Williams to break back and level the set at 3-3.
Petkovic broke again and served for the match at 5-3, only for Williams to break her again and then hold to knot the set at 5-5.
But Petkovic won the last two games to seal the win after one hour and 49 minutes.
"She played really well, and I wasn't as on today as in previous days," Williams said. "But you're going to have days like that and have to work through them.
"I came close, but she was just too good. She's been having a great year and she managed to play those last few games just a little bit better."
Earlier Friday, third-seeded German Angelique Kerber steamrolled Spain's Garbine Muguruza 6-2, 6-1 to set up a semi-final clash with fellow left-hander Varvara Lepchenko.
Kerber broke Muguruza in all seven of her service games and wrapped up the win in 56 minutes.
Unseeded American Lepchenko, who ousted second-seeded Agnieszka Radwanska in the second round, needed only 51 minutes to dismiss US qualifier Sachia Vickery 6-1, 6-0.