Henin, who was honored Thursday as the 2007 WTA Tour Player of the Year, coasted past Angelique Kerber 6-4, 6-2. Williams had no problems disposing of Edina Gallovits 6-1, 6-2, opening her bid to win the tournament for the fifth time in seven years.
Henin will next play Sara Errani.
Third-seeded Svetlana Kuznetsova advanced with a 6-1, 3-6, 6-3 victory over Yung-Jan Chan and No. 11 seed Patty Schnyder fought off Vania King 7-6 (10-8), 4-6, 6-2. Kuznetsova won this event two years ago and fell in the finals last week in Indian Wells to Ana Ivanovic.