NEW YORK -- Defending champion Naomi Osaka advanced to the third round at the US Open on Wednesday after the withdrawal of her second-round opponent, Olga Danilovic, and Simona Halep is into the third round for the first time since 2016.
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.
Twelfth-seeded Halep rolled to a 6-3, 6-1 victory over Kristina Kucova. Halep has won two Grand Slam titles but struggled at the US Open. She lost in the first round in consecutive appearances in 2017 and 2018 and hadn't gone past the second round since reaching the quarterfinals in 2016.
But Halep has made a strong return after missing the French Open and Wimbledon with a calf injury.
"Already it's a better result than when I was healthy, so it's a good thing to be in the third round here," she joked.
Later in the session, Barbora Krejcikova, the French Open champion, reserved a spot in the third round after a 6-3, 6-1 victory over 119th-ranked American Christina McHale.
Krejcikova, who never had appeared in the main draw for singles at Flushing Meadows until this week, won a unique match that began on Court 13 and then shifted to Louis Armstrong Stadium -- which has a retractable roof -- after a heavy downpour arrived in the evening and threatened to wipe out action on the uncovered arenas for the rest of the night.
The eighth-seeded Krejcikova came into 2021 with a pair of Grand Slam doubles championships but only three appearances at majors in singles.
The tournament -- as Wednesday showed -- could be filled with changes and delays amid heavy rain, occasional thunderstorms and the potential for flooding. Only two courts, Arthur Ashe and Louis Armstrong Stadium, have roofs. Play did start on the outside courts Wednesday after they were dried following morning rain before being suspended again.
Among the winners Wednesday were Garbine Muguruza and Ons Jabeur, who both reached the third round with quick victories.
Ninth-seeded Muguruza, a two-time Grand Slam champion, beat Andrea Petkovic 6-4, 6-2 and will next face No. 18 seed Victoria Azarenka, who reached her third US Open final last year and topped Jasmine Paolini 6-3, 7-6 (1) on Wednesday.
"I think it's going to be a very difficult match. I think she always plays great here," Muguruza said. "I think that's the best. Just to go out there and play with people that are top level."
Jabeur, who is seeded 20th, routed Maria Camila Osorio Serrano 6-0, 6-1 in 53 minutes, No. 5 Elina Svitolina defeated Rebeka Masarova 6-2, 7-5, and No. 15 Elise Mertens beat Valentini Grammatikopoulou 6-2, 6-2
The Associated Press contributed to this report.