Garbine Muguruza: Wimbledon champion retires at China Open feeling unwell
World number one Garbine Muguruza retired feeling unwell at the China Open as US Open winner Sloane Stephens was again defeated in the first round.
Spaniard Muguruza, 23, lost the first set 6-1 to unseeded Barbora Strycova, 31, from the Czech Republic.
Wimbledon champion Muguruza then called a medical timeout at 2-0 down in the second set and retired after having her pulse taken courtside in Beijing.
"It's very tough, I got a virus in Wuhan," said the 2015 Beijing champion.
"I couldn't perform today. It's the easy way to think 'don't go on the court'. 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."
Fifteenth seed Stephens lost 6-3 6-0 to qualifier and fellow American Christina McHale in just 62 minutes.
The defeat by the world number 79 extends Stephens' poor run since clinching her first Grand Slam last month.
Last week she also fell at the first hurdle, to China's world number 52 Wang Qiang in the Wuhan Open.
"Tough day, not really much to say," said the world number 17.
"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)."
WTA Finals countdown - Konta hopes in the balance
Stephens' surprise defeat helps Britain's Johanna Konta in her quest to qualify for the WTA finals in Singapore later this month.
Konta lost her fifth straight match on Sunday as she went down 6-1 6-2 to world number 65 Monica Niculescu, damaging her hopes of reaching the tour's showpiece finale.
However, Stephens, CoCo Vandeweghe of the US and France's Caroline Garcia were the only players able to overtake Konta in Beijing, with just two weeks of smaller tournaments to follow.
Vandeweghe remains in contention after defeating Donna Vekic 6-2 6-2 in the first round on Monday, while Garcia - who has a bye into the second round - will play Belgian Elise Mertens on Tuesday.
Former world number one Angelique Kerber needed to win the title to keep her hopes alive but the German lost 6-4 6-4 to France's Alize Cornet in round one.
Anastasia Pavlyuchenkova and Ashleigh Barty must also win in Beijing to have a chance of overtaking Konta before the end of the regular season, as does Elena Vesnina, who is through to round two.