Top seed Agnieszka Radwanska survived a fright in the quarter-finals of the Bank of the West Classic in Stanford.
The Polish player eventually came through 7-6 (7/2) 3-6 6-3 against Uzbekistan-born American Varvara Lepchenko to set up a clash with fourth seed Jamie Hampton in the last four.
Radwanska was shaky on serve and looked to be in danger of following sister Urszula out of the tournament when she dropped the second set, before picking up her game to go through.
American Hampton was more clinical, seeing off Russian Vera Dushevina 6-4 6-3.
Although Radwanska has beaten Hampton in four of their past five meetings, the last time they met, at Eastbourne in June, saw Hampton collect her lone win in their head-to-head series.
Romanian Sorana Cirstea will tackle Dominika Cibulkova in the other semi-final, after they each posted two-set wins.
Fifth seed Cirstea fended off Olga Govortsova of Belarus 6-3 6-2, while Slovakian third seed Cibulkova beat Urszula Radwanska 7-5 6-3.