Williams, ranked 47th in the world, had five breaks in a 6-3, 6-3 win over Garbine Muguruza. Williams dropped serve twice but won nearly two-thirds of the points when serving and 29-of-57 points off Muguruza's serve.
Second-seeded Ana Ivanovic also took half the points when receiving serve as she beat Kurumi Nara 6-2, 6-3 in her quarterfinal. Ivanovic cashed in four of her 10 break points with three breaks coming in the first set.
No. 3-seeded Kristen Flipkens topped qualifier Sachie Ishizu 6-4, 7-5 with three breaks in each set and fifth-seeded Jamie Hampton got by Lauren Davis 6-2, 4-6, 6-4.
Hampton's win sets up an all-American semifinal against Williams. It will be the first meeting between the two. Flipkens, who takes on Ivanovic in the first semifinal Friday, was a straight-set winner over Ivanovic in 2012 in their only previous match.