World number seven Kiki Bertens has become the latest player to pull out of the US Open.
The Dutch government requires people returning from the United States to quarantine for two weeks.
The 28-year-old said that would disturb preparations for her "beloved clay court tournaments".
World number one Ashleigh Barty is the other women's top 20 player to have withdrawn, although number two Simona Halep is not expected to play either.
The tournament is scheduled to begin on 31 August.