Li, coming back from a right knee injury which forced her to miss the Australian Open and start the season less than a month ago, followed up her knockout of top-seeded Agnieszka Radwanska of Poland with an easier win over Cetkovska, breaking the Czech three times.
Li will face another Czech in Friday's quarterfinals, Lucie Safarova, who advanced when eighth-seeded Maria Kirilenko of Russia retired with an inflamed left knee while Safarova was leading 6-4, 4-1.
Seventh-seeded Gisela Dulko of Argentina beat Mariya Koryttseva of Ukraine 6-2, 6-4 for her second quarterfinal berth in three weeks, and awaited the winner of a late match between fourth-seeded Zheng Jie of China and Melinda Czink of Hungary.
