(Reuters) – World number one Garbine Muguruza fell at the first hurdle at the China Open on Monday, with the Spaniard forced to retire during her first-round match against Czech Barbora Strycova.

Muguruza looked sluggish throughout and was broken in the first game before losing the first set 6-1. She was trailing 2-0 in the second when she decided she could not continue.

“I got a virus in Wuhan and I did not practise in Beijing. I thought an extra day’s rest would help, and didn’t want to pull out,” Muguruza said.“ But I couldn’t perform today.”

American qualifier Christina McHale served up another shock when she beat Sloane Stephens 6-3 6-0 to send the U.S. Open champion crashing out in the first round of a tournament for the second week in a row.

Stephens, who is seeded 15th in Beijing, was eliminated in the first round of last week’s Wuhan Open by China’s Wang Qiang.

Peng Shuai had to dig deep to get past American Shelby Rogers, but the Chinese edged out her opponent 7-6(3) 3-6 7-5 in an epic match lasting two hours and 48 minutes.

Shuai faces Romanian Monica Niculescu in the second round.