Belinda Bencic, a US Open semi-finalist 12 months ago, has become the latest player to pull out of this year's tournament.
The 23-year-old from Switzerland, who lost to eventual champion Bianca Andreescu, has confirmed she will not be travelling to New York later this month.
She wrote on Twitter: "I have made the difficult decision to skip the US swing in New York and will return to the starting tour in Rome next month."
Her decision means three of last year's semi-finalists - Bencic, Andreescu and Elina Svitolina - have withdrawn, meaning the only one of the four competing this year will be Serena Williams.
The world number one, Ashleigh Barty, also pulled out because of her concerns surrounding coronavirus.
The Cincinnati Masters, also being held at Flushing Meadows, begins on August 21 with the US Open beginning a week later.
- TEAMtalk media