World number one Garbine Muguruza retired ill from the first round of the China Open on Monday and US Open winner Sloane Stephens went out after suffering a shock defeat.
The Spaniard Muguruza suggested in the build-up that she was not fully fit after a leg injury 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.
Stephens meanwhile 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 in Beijing. 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 hurdle, losing to China's Wang Qiang in the opening round of 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).'
Asked if the delayed start for rain had disrupted her, she said: 'Yes, but it affects both players, so... I mean, she obviously just handled it a little bit better.
'It's unfortunate, but it happens.'