Williams, the event's defending champion, defeated Anabel Medina Garrigues 3-6, 6-0, 7-5 by winning five of the last six games. One day after she lost only two points on serve in the third round, Williams serve was broken five times Friday.
That included a pair of third-set breaks that put Medina Garrigues, who won four consecutive games, up 4-2 in the final set. Williams quickly broke Medina Garrigues' serve and then posted another break to go up 6-5. She served out the match, winning the final game at love.
Second-seeded Maria Sharapova won nearly 70 percent of the points on serve in a 6-2, 6-4 win over Kaia Kanepi. Sharapova lost her serve once but also cashed in four of her 12 break chances.
Williams draws Errani, against whom she is 4-0 in her career, in Saturday's semifinals. The all-Russian other semifinal sees Sharapova against Ivanovic. Sharapova is 6-2 lifetime versus Ivanovic, including a three-set win earlier this year in Germany.