(Reuters) - World number one Simona Halep was forced to retire in the first round of the China Open on Sunday after aggravating a back injury.

FILE PHOTO: Tennis - Wimbledon - All England Lawn Tennis and Croquet Club, London, Britain - July 5, 2018 Romania's Simona Halep celebrates winning the second round match against China's Saisai Zheng REUTERS/Andrew Couldridge/File Photo

The Romanian injured herself in training at last week’s Wuhan Open where she crashed out in the second round after being beaten in straight sets by Slovak Dominika Cibulkova.

Halep lost the first set to Tunisia’s Ons Jabeur 6-1 on Sunday before deciding she could not continue and said she would have a scan to assess the full extent of the damage.

“There is a pain,” Halep added. “I don’t know exactly what it is... I will have an MRI and I will see. I’m just worried now. I feel sad I couldn’t finish. That’s why I stopped, because I felt like it’s getting worse if I continue.”

Wimbledon champion and third seed Angelique Kerber crushed France’s Kristina Mladenovic 6-2 6-2 to cruise into the second round.

The German won 77 percent of her first serves and struck 17 clear winners as Mladenovic wilted in the face of her aggressive brand of tennis.

Defending champion Caroline Garcia survived a marathon encounter against Chinese wildcard Wang Yafan that stretched over three hours to scrape into the second round with a 7-6(10) 6-7(4) 6-3 victory.

Garcia, who was also defending champion at the Wuhan Open but lost her opening match to Katerina Siniakova last week, said she was relieved to progress.

“I was definitely very upset after Wuhan,” the Frenchwoman added. “Last week against Siniakova, I think it was overall a better match, but it was the same -- I was up, I couldn’t finish. I’m happy I found the way.”

American Sloane Stephens bounced back after losing a first-set tiebreak to beat Russian Anastasia Pavlyuchenkova 6-7(8) 6-4 6-4 in the day’s late match.

Stephens, the 2017 U.S. Open champion, had not won a match in Asia since the 2015 China Open, but sent down 39 winners to win in two hours and 53 minutes.

Next up for the American is a meeting with China’s Zheng Saisai, who beat Ukrainian Dayana Yastremska 6-4 6-3 earlier in the day.

“It’s one at a time right now,” Stephens said. “It’s too stressful to think about, so I’ll just take one match at a time, and see what happens.”