Iga Swiatek, top seed, debuted at the Tokyo tennis tournament with a two-set victory, 6-4 and 7-5, over local player Mai Hontama, ranked 148 in the world.
The Polish player had not played since the US Open, when she lost to Latvian Jelena Ostapenko in the round of 16.
Against Hontama, the former world number one had problems with her serve, which she lost twice in the opening set and three times in the second, forcing her to play catch-up.
After wasting two match points at 5-4, she comfortably won the next two games. Her next opponent in the quarterfinals will be Russian Veronika Kudermetova, the eighth seed, who defeated American Kayla Day 6-3 and 6-3.
American Jessica Pegula also advanced to the quarterfinals, defeating Spanish player Cristina Bucsa 6-1 and 6-2, while Russian Ekaterina Alexandrova emerged victorious over compatriot Luidmila Samsonova 6-4 and 6-2.