Marion Bartoli of France knocked holder Anna Chakvetadze out of the Stanford Classic 6-3, 6-4 in the quarter-finals on Friday, while top seed Serena Williams brushed aside Patty Schnyder 6-3, 6-1.
Bartoli will face Ai Sugiyama in the semi-finals after the Japanese fought back to beat Slovakian Dominika Cibulkova, who retired down 6-7 7-6 5-3.
Bartoli found success by attacking the Chakvetadze serve, breaking the second seed five times.
"I just didn't feel well," said the Russian. "It was tough to serve with the heavy balls and my movement was a little slow. But she played solid."
Williams registered five double faults in her match but still outgunned her opponent in less than an hour.
"You have to be quick against Patty because she's such a big shotmaker you never know what's going to happen," said Williams, who will now face Canada's Alexandra Wozniak.
Wozniak reached her first Tier II WTA semi-final with a gutsy 6-2 5-7 6-4 victory over Australian Samantha Stosur.
After losing the first set, Japan's Sugiyama found herself 5-4 down against the 19-year-old Cibulkova in the second. However, the 33-year-old Japanese player mounted a stunning comeback and took full advantage when Cibulkova suffered cramp early in the third set.
Cibulkova received a medical time out down 5-3 in the third set but could not finish the match.