Danilovic, a qualifier who is ranked No. 145, wrote on social media that she had “a non-Covid related viral illness.”
Osaka has twice won the US Open title, in 2018 and 2020. She is seeking her fifth major title in New York and will next play the winner of Wednesday’s Leylah Fernandez– Kaia Kanepi match for a spot in the fourth round.
The tournament will likely be filled with changes and delays Wednesday as heavy rain, occasional thunderstorms and the potential for flooding is in the forecast. Only two courts, Ashe and Louis Armstrong Stadium, have roofs.
Play did start on the outside courts Wednesday after they were dried following morning rain.
The Associated Press contributed to this report.