Former champion Bianca Andreescu (23) has withdrawn from the US Open due to injury, organisers said on Saturday, after the Canadian suffered a stress fracture in her back.
Andreescu, who won the tournament in 2019, lost her opening match at the Citi Open in Washington DC last month before going down in straight sets to Camila Giorgi (31) at the Canadian Open.
She appeared to be in discomfort against Italian Giorgi and needed to call a physiotherapist on court.
Andreescu then withdrew from the Cincinnati Masters, saying: "During my match in Washington I started experiencing pain in my back that kept getting worse and I did everything I could the other night in Montreal to play at my highest level.
"All of this led me to undergo medical examinations and sadly the results have revealed a small stress fracture in my back."
Andreescu was scheduled to face Ukraine's Lesia Tsurenko (34) in the US Open first round.