Tanasugarn, who has played mostly on the ITF Tour since winning in Osaka last year, beat Mathilde Johansson 6-2, 6-4 in a second-round match. Tanasugarn had a solid service game, dropping serve once and winning 71 percent of the points on serve.
She next goes against third-seeded Angelique Kerber, who was a 6-1, 6-3 winner over Olga Govortsova in her second-round match. Kerber had six service breaks in eight receiving games.
Zheng Jie posted the upset of the day with a 7-5, 6-3 victory over fifth-seeded Jarmila Gajdosova. Zheng won as many points -- 31-31 -- as Gajdosova when Gajdosova served. That led to five service breaks for Zheng.
No. 4-seeded Petra Cetkovska blew past Erika Sema 6-0, 6-3 in 60 minutes. Cetkovska won 70 percent of the points on serve.
Cetkovska and Zheng will play in a quarterfinal Friday.