Women's world No. 5 Elina Svitolina and Kiki Bertens, ranked seventh, joined a growing list of players withdrawing from this month's US Open due to the coronavirus pandemic on Friday.
Men's defending champion Rafael Nadal and women's world No. 1 Ashleigh Barty have opted against travelling to New York to play the hardcourt Grand Slam as well as the Western & Southern Open that will work as a warm up for the US Open.
"I understand and respect all the efforts they are putting to make it happen in a safe environment, but I still don't feel comfortable to travel to U.S. without putting my team and myself at high risk," Ukrainian Svitolina, who reached last year's semifinals in New York, said on Twitter.
The United States has close to five million coronavirus cases and almost 160,000 people have died, according to a Reuters tally.
"The situation around COVID-19 is still worrying and the health of everyone and the control over this virus is priority," Bertens said on Instagram.
The US Open organisers (USTA) are creating a bio-secure "bubble" in New York to hold the tournament from Aug. 31 without spectators. But concerns remain about whether players would face mandatory quarantines upon returning home from the US Open.
Former world No. 1 Andy Murray, who won the US Open in 2012, said on Monday that players travelling to New York needed assurance that they would not face mandatory quarantines after returning to Europe.
"Our prime minister indicated yesterday that we should be quarantined for 14 days after coming back from the States," Bertens added.
"Of course we respect this as a team and this would disturb our preparation for my beloved clay court tournaments in Rome and Paris."
The Italian Open is scheduled to be held in Rome from Sept. 20 ahead of the French Open Grand Slam in Paris a week later.