Defending champion Naomi Osaka advanced to the third round of the US Open by walkover on Wednesday as rain delayed the start of play on the New York hardcourts.

The Japanese third seed, seeking her fifth Grand Slam title and third US Open crown in four years, moved on when 20-year-old Serbian qualifier Olga Danilovic withdrew for an undisclosed medical reason.

Osaka, 23, is trying to become the first back-to-back US Open winner since Serena Williams, absent with a torn hamstring, captured her third in a row in 2014.

Osaka's Grand Slam win streak remains at 16 matches, the walkover not counting toward the run just as her withdrawing from the French Open after the first round did not halt the run.

Next up for Osaka will be the winner of a later match between Estonia's Kaia Kanepi and Canadian Leylah Fernandez.

The remnants of Hurricane Ida, which slammed into the US Gulf Coast at the weekend, drenched the grounds to postpone outside court matches.

But the show went on under retractable roof covering at Arthur Ashe Stadium and Louis Armstrong Stadium.

Osaka was to have played first on Ashe but her match was replaced by Romanian 12th seed Simona Halep facing Slovakian Kristina Kucova.

Russian second seed Daniil Medvedev, this year's Australian Open runner-up, was set to follow Osaka into Ashe to face Germany's Dominik Koepfer.

Medvedev, a winner last month in Toronto, won his only prior match against the left-hander on his way to the 2019 US Open final, which he lost to Rafael Nadal.

Stefanos Tsitsipas, the Greek third seed whose extended bathroom breaks have become the subject of ridicule at the Open, has the last Ashe night match against Frenchman Adrian Mannarino, who is 0-17 in Grand Slams against top-10 opponents.

Tsitsipas was derided by three-time Grand Slam champion Andy Murray and Olympic champion Alexander Zverev for taking long toilet breaks and other pauses to disrupt opponents.

Murray says he "lost respect" for the French Open runner-up and Zverev joked the Greek star travels to the moon to use the bathroom.

Tsitsipas, who would match his best US Open run by reaching the third round, says he's only using rules available to all ATP Tour players.

"I'm playing by the rules and sticking to what the ATP says is fair," Tsitsipas said.

Other women's matches set for indoor starts include 2020 US Open runner-up and two-time Australian Open champion Victoria Azarenka against Italy's Jasmine Paolini and the night feature at Ashe between American stars Coco Gauff and Sloane Stephens, the 2017 US Open winner.

Among those hopeful of making a start when the courts are dry were Russian fifth Andrey Rublev, set to meet Spaniard Pedro Martinez; Belarusian second seed Aryna Sabalenka, scheduled to play Slovenia's Tamara Zidansek, and Ukraine's fifth-seeded Elina Svitolina, who meets Spain's Rebeka Masarova.