World number one Garbine Muguruza retired ill with a virus in the first round of the China Open on Monday and US Open winner Sloane Stephens went out in a shock defeat.

Spain's Muguruza suggested in the build-up that she was not fully fit and she lost the first set 6-1 to unseeded Barbora Strycova of the Czech Republic.

Muguruza, 23, the reigning Wimbledon champion, called a medical timeout at 2-0 down in the second set and then called it quits after having her pulse taken on the side of the court.

Muguruza said afterwards she had been too unwell even to practise for her opener but had been determined to play, having lost in the quarter-finals last week in Wuhan, where she struggled with a leg injury.

"Since I'm in Beijing, I didn't play at all. I don't know, I got a virus in Wuhan," she said.

"I want to feel like giving the last chance, going on the court, just hit a few balls, see how I'm going to feel, try to turn things around.

"I don't want to retire before the tournament starts for me."

Muguruza, yet to win a tournament since reaching the top ranking in mid-September, added: "It's good that I could play on Monday, I had more days to rest.

"But in fact, I couldn't perform today."

Strycova faces Julia Goerges of Germany in the second round.

Muguruza headed for the exit soon after Stephens tumbled out in the first round for the second week running, this time at the hands of qualifier Christina McHale.

The 15th seed Stephens, wearing all pink, saw her match delayed by rain falling on Beijing's outdoor hardcourts.

When it finally got started she failed to fire, going out to her fellow American with a whimper, 6-3, 6-0 in just 62 minutes.

The loss extends Stephens' poor run after sealing her first Grand Slam last month.

Last week the 24-year-old similarly fell at the first, losing to China's Wang Qiang at the Wuhan Open.

Stephens, ranked 17 in the world to McHale's 79, refused to make any excuses during a post-match press conference lasting less than 90 seconds.

"Tough day, not really much to say," she said.

"Obviously not a great match so just forget about it and move on, try to have a better level for the next tournament (Hong Kong)."

The comeback of the tournament so far belonged to the Belgian Elise Mertens.

The 21-year-old recovered from 5-0 in the first set to trump the eighth seed Dominika Cibulkova 7-6 (7/4), 6-1.

The second seed Simona Halep saw her second-round match against Magdalena Rybarikova shelved after the morning drizzle.

- Nadal primed for opener -

In the men's draw, top seed and world number one Rafael Nadal begins his China Open quest on Tuesday against Lucas Pouille.

The 23-year-old Frenchman has lost his last three matches on the trot, but he beat Nadal at the US Open last year -- the last time they met -- and the Spanish legend anticipates a stern test.

"He has a great level of tennis. Hopefully he will not finish the season that well because I play tomorrow against him," Nadal said, smiling.

"But I am sure he will be fighting for important things next year, he has the talent to do it.

"He has the game, the right level of tennis to make that happen if he wants to do it."

Source: AFP