Williams, reaching her third semis in 2013 and looking for her first tournament title since 2010, got by Eugenia Bouchard 6-3, 6-7 (4-7), 6-3. Williams lost her serve four times in the match while converting eight of her 18 break-point chances.
She was good on just 50 percent of her first serves and had seven double faults but used the strong receiving game to advance. The match took 3 hours, 2 minutes to complete.
While Williams is unseeded, the other three semifinalists are among the first seven seeds this week. Williams draws seventh-seeded Petra Kvitova in one semifinal while fourth-seeded Carolina Wozniacki plays No. 5 Angelique Kerber in the other.
Kerber knocked off No. 2-seeded Agnieszka Radwanska 6-4, 6-4, losing just 11 points and one game in 10 games serving. Wozniacki came from behind for a 2-6, 6-3, 6-2 decision over Lucie Safarova, seeing only one break point in the final two sets. Kvitova was a 6-4, 6-1 winner over Svetlana Kuznetsova, saving all six break points she faced.
Williams takes on Kvitova in the first semifinal Friday. They have split their two previous meetings with both going three sets. Wozniacki and Kerber are 3-3 against each other in their careers. Wozniacki took a three-set win earlier this year in California.