Venus Williams upset top-seeded Victoria Azarenka 6-2, 6-4 on Tuesday to advance to the third round of the Pan Pacific Open.
Williams, making her first appearance in the Tokyo tournament since 2009, took advantage of six-double faults by Azarenka, who struggled with her game from the outset at Ariake Colosseum.
Williams next faces No. 13-seeded Simona Halep of Romania, who defeated Germany's Andrea Petkovic 7-6 (4), 6-3.
In other second-round matches, second-seeded Agnieszka Radwanska of Poland defeated Canada's Aleksandra Wozniak 7-6 (5), 6-0. Fourth-seeded Caroline Wozniacki of Denmark overpowered Flavia Pennetta 6-2, 6-3.
Svetlana Kuznetsova also advanced by beating third-seeded Sara Errani of Italy 6-4, 6-4.
Kuznetsova, a two-time major winner, hit seven aces in the match. The Russian and will next face 15th-seeded Sorana Cirstea of Romania. Cirstea beat Japanese wild-card entry Misaki Doi 6-1, 6-2.
Canada's Eugenie Bouchard defeated ninth-seeded Sloane Stephens of the United States 5-7, 7-6 (7), 6-3. Bouchard will next play sixth-seeded Jelena Jankovic of Serbia, who beat Japan's Ayumi Morita 6-4, 6-1.
Other winners include Samantha Stosur, Ana Ivanovic, Dominika Cibulkova and Madison Keys.