Ana Ivanovic stalled Victoria Azarenka's comeback from a foot injury with a 6-3, 6-4 win to make the Pan Pacific Open quarter-finals.
In the match between the former major champions and number ones, Azarenka committed seven of her nine double faults in the first set and her serve was easily read by Ivanovic.
Ivanovic prevailed for her first win over Azarenka since 2010, after losing four straight. She lined up seventh-seeded Lucie Safarova of the Czech Republic, who hit 10 aces in beating wild card Belinda Bencic of Switzerland 3-6, 6-4, 6-2.
Top-seeded Angelique Kerber of Germany beat Elina Svitolina of Ukraine 6-4, 6-4. Kerber, who is seeking her fourth career WTA singles title, converted both break points in the second set at Ariake Coliseum.
On Friday, she will play sixth-seeded Dominika Cibulkova of Slovakia, who ousted Coco Vandeweghe of the United States, 7-6, 7-5, in a match of 15 broken serves.
Elsewhere, tpp-seeded Agnieszka Radwanska thrashed Chanelle Scheepers 6-0, 6-0 in the second round of the Korea Open.
Defending champion Radwanska lined up fifth-seeded Varvara Lepchenko of the United States, who ousted South Korean wild card Han Na-Lae 7-5, 6-1.