Holder Anna Chakvetadze crashed out of the Stanford Classic 6-3 6-4 in the quarter-finals to French ace Marion Bartoli.
The Russian found it difficult to serve in the blustery conditions and Bartoli capitalised, breaking the second seed five times.
But Chakvetadze was gracious in defeat and praised her opponent.
She said: "I just didn't feel well. It was tough to serve with the heavy balls and my movement was a little slow. But she played solid."
Bartoli faces Ai Sugiyama in the semis after the Japanese player's quarter-final opponent, Slovakian Dominika Cibulkova, retired when 6-7 7-6 5-3 down.
Sugiyama had to battle back from one set down and 5-4 down in the second against her19-year-old opponent.
But she held her nerve and took full advantage when Cibulkova suffered cramp early in the third set. The Slovakian then received a medical time out at 5-3 down but could not finish the match.
In the other semi-final, top seed Serena Williams will play Canada's Alexandra Wozniak.
Despite giving away five double faults, the American beat Patty Schnyder 6-3 6-1 in less than an hour.
Welcoming her win, Williams said: "You have to be quick against Patty because she's such a big shotmaker you never know what's going to happen."
Wozniak reached her first Tier II WTA semi-final with a gutsy 6-2 5-7 6-4 victory over Australian Samantha Stosur.

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