Petra Kvitova and Angelique Kerber were among the second-round winners Monday at the Pan Pacific Open.
The seventh-seeded Kvitova earned a 7-5, 6-4 triumph over Swiss wild card Belinda Bencic, while the fifth-seeded Kerber thumped Spain's Maria-Teresa Torro-Flor, 6-0, 6-1.
One other second-round match saw Czech veteran Lucie Safarova post a 7-5, 6-4 upset of eighth-seeded Roberta Vinci.
Each of the top eight seeds received byes into the second round.
The first round was completed Monday and seeded winners included Ana Ivanovic and Simona Halep.
Ivanovic, seeded 11th this week, dropped the first game in each set against Annika Beck, but roared back each time and completed a 6-1, 6-1 rout of the German in just 50 minutes. Halep had rallied for a 3-6, 7-6 (7-5), 3-0 lead against Anastasia Pavlyuchenkova before the Russian retired in the third set. Pavlyuchenkova was playing just a day after falling to Agnieszka Radwanska in the Seoul final.
American Madison Keys pulled off a 5-7, 7-5, 6-2 upset of 10th-seeded Spaniard Carla Suarez Navarro, while Italy's Flavia Pennetta took the court for the first time since reaching the U.S. Open semifinals and came away with a 6-3, 6-3 win over Slovakia's Daniela Hantuchova, a quarterfinalist at Flushing Meadows.
Three Japanese natives also thrilled the home crowd with wins, led by 1995 Pan Pacific champ Kimiko Date-Krumm. The Tokyo resident, who will turn 43 later this week, held a 6-2, 4-1 lead over Anastasia Rodionova when the Australian qualifier retired with an abdominal injury.
Ayumi Morita also prevailed with a 7-6 (7-4), 6-3 win over Britain's Laura Robson and Misaki Doi rallied for a 6-7 (3-7), 6-0, 7-5 triumph over American Varvara Lepchenko.
China's Peng Shuai disappointed the Tokyo crowd with a 6-2, 6-1 throttling of qualifier Risa Ozaki, while former French Open champ Francesca Schiavone dropped a three-setter to Canada's Aleksandra Wozniak.
Other unseeded winners were Slovakia's Magdalena Rybarikova, Australia's Casey Dellacqua, Canada's Eugenie Bouchard and Germany's Andrea Petkovic.