Bianca Andreescu will not defend her US Open title, becoming the latest top-10 women's player to skip the grand slam event at Flushing Meadows.
World No.6 Andreescu's withdrawal from the August 31-September 13 tournament follows that of Australian Ashleigh Barty (No.1), Ukraine's Elina Svitolina (No.5) and Dutchwoman Kiki Bertens (No.7).
The 20-year-old Canadian, who has not competed since she suffered a knee injury last October at the WTA Finals in Shenzhen, said the COVID-19 pandemic had compromised her ability to reach her highest level of form.
"After many discussions with those closest to me, I have made the difficult decision not to return to New York this year," said Andreescu, who beat Serena Williams last year to become Canada's first tennis grand slam singles champion.
"I have taken this step in order to focus on my match fitness and ensure that I return ready to play at my highest level."
Her decision also means the grand slam will be missing both reigning singles champions as men's world No.2 Rafael Nadal previously said he would skip the event.
"The US Open victory last year has been the high point of my career this far and I will miss not being there," said Andreescu.
"However, I realise that the unforeseen challenges, including the COVID pandemic, have compromised my ability to prepare and compete to the degree necessary to play at my highest level."
The United States Tennis Association decided to go ahead with the grand slam without spectators while players will be asked to follow strict measures, including regular testing, in a bid to avoid spread of the virus.
"I want to express my appreciation to the USTA and the WTA for all of their efforts in making the event happen," said Andreescu. "I look forward to joining my competitors back on court soon."