Top-ranked Simona Halep retired Sunday from her first-round match at the China Open.
Halep lost the first set 6-1 to Tunisia's Ons Jabeur before retiring with a back injury that she also had problems with at last week's Wuhan Open.
It's the fourth straight loss for the defending French Open champion.
Third-seeded Angelique Kerber of Germany cruised past Kristina Mladenovic 6-2, 6-2 to reach the second round. The Wimbledon champion broke Mladenovic four times to extend her perfect record in opening-round matches at Beijing to nine.
Defending champion Caroline Garcia of France survived an early scare against Chinese wildcard Wang Yafan to win 7-6 (10), 6-7 (4), 6-3.
Americans Madison Keys and Sloane Stephens both won their matches to advance, while CoCo Vandeweghe was beaten by Slovenian qualifier Polona Hercog 6-2, 6-1.
Laura Siegemund defeated No. 13 Daria Kasatkina 6-3, 4-2 after Kasatkina retired with gastrointestinal illness.
Timea Babos, Zheng Saisai, Lesla Tsurenko, Dominika Cibulkova and Kirsten Flipkens also advanced.
