The 2018 US Open winner survived a scare as she overcame the 18-year-old, the world number 137, 6-3, 6-7 6-2 on Arthur Ashe Stadium at Flushing Meadows.

Osaka will face Anett Kontaveit of Estonia, the 14th seed who beat Poland's Magda Linette 6-3, 6-2, in round four.

Victory for the Japanese star, seeded fourth, came during a session largely overshadowed by confusion surrounding Alexander Zverev of Germany's third-round encounter with Frenchman Adrian Mannarino.

The match was hit by a near three-hour delay amid issues regarding Mannarino's status in what has been described as the "bubble within the bubble".

Mannarino is among the 11 players placed under additional restrictions after coming into contact with compatriot Benoît Paire, who tested positive for COVID-19 on the eve of the tournament.

Zverev, the fifth seed, was pictured shirtless in the stands when he was supposed to be on court with his French opponent.

His brother, Mischa Zverev, told Eurosport Germany that the match had been delayed while they waited for "a decision from somebody".

Reports suggested the United States Tennis Association (USTA) was awaiting a decision from New York Governor Andrew Cuomo on whether Mannarino would be able to play.

A statement from the USTA failed to shed any light on the situation.

"The Zverev-Mannarino match was delayed while a collaborative dialogue with health officials was conducted today," the statement read.

"Communication with the players was ongoing during the afternoon to keep them updated at all times.

"Given the sensitivity of the medical issues involved, the USTA is not able to provide further details."

The match on Arthur Ashe eventually got underway two hours and 45 minutes later than scheduled.

Players competing at the event in New York, one of the hardest-hit American states by the coronavirus pandemic, have to adhere to strict measures while in the "bubble".

Elsewhere in the men's draw, Alejandro Davidovich Fokina of Spain continued his dream run at the tournament by beating Britain's Cameron Norrie 7-6, 4-6, 6-2, 6-1.

Australian Jordan Thompson reached the fourth round at a Grand Slam for the first time after defeating Mikhail Kukushkin of Kazakhstan 7-5, 6-4, 6-1.

Germany's Angelique Kerber, another former US Open champion, is also through after she outclassed Ann Li of the United States.

The three-time Grand Slam champion, who lifted the US Open women's singles crown in 2016, won 6-3, 6-4 to set up a meeting with Li's compatriot Jennifer Brady.

Brady proved too strong for Caroline Garcia of France, recording a 6-3, 6-3 victory to knock out the conqueror of top seed Karolína Plíšková of the Czech Republic.