World number one and top-seed Simona Halep retired from her first-round contest at the China Open due to a back injury.

Romania's Halep lost the opening set to Tunisian qualifier Ons Jabeur 6-1 in 31 minutes before calling a halt to proceedings.

"There is a pain. I couldn't move properly, so that's why I stopped," Halep said on wtatennis.com.

"I don't know exactly what it is. I will go home. I will have an MRI (scan) and I will see.

"I'm just worried now. I feel sad that I couldn't finish. That's why I stopped, because I felt like it's getting worse if I continue.

"It's really tough when you have back injury because you cannot bend, you cannot react at the balls. I couldn't play."

Third seed Angelique Kerber of Germany had no such trouble, defeating France's Kristina Mladenovic 6-2 6-2, and fourth seed Caroline Garcia of France beat wildcard Wang Yafan 7-6 (12/10) 6-7 (4/7) 6-3.

Zheng Saisai, another Chinese wildcard, advanced after beating Dayana Yastremska 6-4 6-3.

Daria Kastakina, the 13th seed, was 6-4 4-2 down to Germany's Laura Siegemund before retiring.

Slovenia's Polona Hercog, a qualifier, defeated Coco Vandeweghe of the United States 6-2 6-1.

Latvia's Anastasija Sevastova, Lesia Tsurenko of Ukraine, Dominika Cibulkova of Slovakia and USA's Madison Keys were the day's other winners.

USA's Sloane Stephens, the ninth seed, defeated Anastasia Pavlyuchenkova of Russia 6-7 (8/10) 6-4 6-4.

British qualifier Katie Boulter was beaten 4-6 6-2 7-5 by Belgium's Kirsten Flipkens.