Fourth seed Elina Svitolina became the highest-profile casualty in the women's draw at the French Open after a surprise straight-sets defeat.
The Ukrainian lost 6-3 7-5 to Romania's Mihaela Buzarnescu, the world number 33 who had never won a Grand Slam match before this week.
But Australian Open champion Caroline Wozniacki eased into the last 16.
Wozniacki, the second seed at Roland Garros, defeated France's Pauline Parmentier 6-0 6-3.
She will next face Russian 14th seed Darya Kasatkina, who beat Greece's Maria Sakkari 6-1 1-6 6-3.
"Today, really, I was off and couldn't find my rhythm, my game. I was really struggling," said Svitolina.
"On the other hand, she was playing great tennis. She was really, really on the ball."
Play was later cancelled for the day following a heavy spell of rain with eighth seed Petra.
Kvitova and 10th seed Sloane Stephens yet to start their matches.