World number one Garbine Muguruza retired ill with a virus in the first round of the China Open on Monday, while 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 lost the first set 6-1 to unseeded Barbora Strycova of the Czech Republic.
Muguruza, 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.
The 23-year-old said afterwards she had been too unwell even to practice 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."
"It's good that I could play, 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 is yet to win a tournament since reaching the top ranking in mid-September.
Elsewhere, 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 saw her match delayed by rain falling on Beijing's outdoor hardcourts, and when it finally got started she failed to fire, going out to her fellow American, losing 6-3, 6-0 in just 62 minutes.
The loss extends Stephens' poor run after sealing her first Grand Slam last month, and comes just a week after 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 that lasted 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)."
Monday also saw the comeback of the tournament so far as 21-year-old Belgian Elise Mertens recovered from 5-0 in the first set to trump the eighth seed Dominika Cibulkova 7-6 (7/4), 6-1.
There were also first-round victories for Peng Shuai, Elena Vesnina, Coco Vandeweghe and Varvara Lepchenko.
The second seed Simona Halep saw her second-round match against Magdalena Rybarikova shelved after the morning drizzle.