Williams ended the eight-match winning streak of Magdalena Rybarikova with a 6-1, 6-1 victory. The top-seeded Williams is seeking her eighth tournament title of the year in 11 events and will try to take another step toward a championship when she meets Agnieszka Radwanska in Saturday's semifinals.
Radwanska advanced by downing Sara Errani 7-6 (7-1), 7-5.
Williams has played three matches in this week's tour stop and her opponents have won no more than three games in any set. She won almost two-thirds of the points played in the match with Rybarikova, who faced Williams for the first time in her career.
Rybarikova was coming off a tournament title last week in Washington.