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 No.1 and top seed Serena Williams is in action later on 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.
Earlier, 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.
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.