Willliams will face Aleksandra Wozniak in the last four
Serena Williams breezed into the semi-finals of the Stanford Classic with victory over Patty Schnyder.
The American top seed registered five double faults but still won 6-3 6-1 in less than an hour.
Schnyder, who had won three of the last five meetings against Williams, failed to find any sort of form.
"You have to be quick against Patty because she's such a big shotmaker you never know what's going to happen," said Williams.
Defending champion Anna Chakvetadze was sent crashing out in straight sets by Marion Bartoli.
The 23-year-old Frenchwoman, who defeated British number one Anne Keothavong in the second round, won 6-3 6-4 to book a meeting with Ai Sugiyama in the semi-final.
After losing the first set in her quarter-final against Slovakian Dominika Cibulkova, Japan's Sugiyama found herself 5-4 down against the 19-year-old in the second.
However, Cibulkova received a medical time out down 5-3 in the third set but could not finish the match.
Williams will face Alexandra Wozniak in the other semi-final after the Canadian produced a gutsy display to beat Australian Samantha Stosur, 6-2 5-7 6-4.
Venus Williams has withdrawn from next week's Los Angeles Classic with tendonitis in her right knee. She has has not played a tour match since beating sister Serena to win a fifth Wimbledon title two weeks ago.