Top-ranked Angelique Kerber needed three sets to get by France's Kristina Mladenovic 6-7 (4), 6-1, 6-4 in the second round of the Wuhan Open in China on Tuesday, while No. 2 seed Garbine Muguruza was upset by Jelena Jankovic 6-4, 7-6 (2).
It was Kerber's first match since taking over the No. 1 ranking from Serena Williams, who withdrew from the tournament to rest her ailing shoulder. Kerber dropped the opening set, but committed only 25 errors in moving on to a meeting with Petra Kvitova in the next round.
"Here we have a new tournament, a new day tomorrow. I have to play better than today, for sure," Kerber said. "I will try my best to go out there to win the match. But I will have to play my one of my best tennis tomorrow against her."
No. 13 seed Roberta Vinci was also upset by Yaroslava Shvedova 7-5, 6-2.
The other seeded players in action all won, including No. 6 Venus Williams dispatching of Yulia Putintseva 6-3, 6-2 and fellow American Madison Keys beating France's Carolina Garcia 6-3, 6-4. The other Americans in action Tuesday, Madison Brengle and Louisa Chirico, were both straight-sets losers, with Brengle falling 6-0, 6-4 to ninth-seeded Svetlana Kuznetsova and Chirico ousted by fellow qualifier Dariaka Satkina of Russia 6-1, 6-1.
No. 3 seed Agnieszka Radwanska, No. 11 Johanna Konta and unseeded Carolina Wozniacki also breezed into the third round with straight-sets victories, while No. 10 Dominika Cibulkova needed three sets to get by Laura Siegemund 2-6, 6-3, 6-4 and Barbora Strycova outlasted Misaki Doi 7-5, 0-6, 6-2.
Wozniacki is coming off a title at the Toray Pan Pacific Open, and next faces Radwanska for their second meeting in as many weeks.
"We always play good matches, so I'm not expecting it to be easy," Radwanska said. "We just played three hours in Tokyo so we could have another battle in front of us."
Williams, playing for the first time since a grueling three-set loss at the U.S. Open, will take on Kuznetsova.
"I haven't played in three weeks," said Williams. "When you have a lot of experience, the first match, you never feel like guns ready to go as you might feel in the finals.
"At least you know what you're doing well, what you need to improve. You're ready to do it."