Serena Williams remains on course for a record-equalling 24th Grand Slam singles title after recovering from a slow start to beat Sloane Stephens in the US Open third round on Saturday.
But Day 6 was dominated by fresh controversy after New York health officials ordered the women's top doubles pairing be withdrawn over coronavirus restrictions.
Williams came from a set down to oust 2017 champion Sloane Stephens 2-6, 6-2, 6-2, in what she called an "intense" encounter on Arthur Ashe Stadium.
"In that first set I don't think she made any errors, honestly. She was just playing so clean and I just said, 'I don't want to lose in straight sets,'" said Williams, who will play 15th seed Maria Sakkari for a place in the quarter-finals.
The Greek player, who stunned Williams at the Western & Southern Open last month, progressed with a 6-3, 6-1 dismantling of America's 22nd seed Amanda Anisimova.
Off the court, No 1 seeds Kristina Mladenovic and Timea Babos were pulled from the women's doubles competition after Mladenovic was instructed to quarantine for coming in contact with French compatriot Benoit Paire, who tested positive for Covid-19 last weekend.
"All persons who were identified as having prolonged close contact with the infected player will quarantine in their rooms for the remainder of their quarantine period," the United States Tennis Association said in a statement.
The USTA said local public health authorities had issued the quarantine notices, which prevented the players commuting from their hotel to the USTA Billie Jean King National Tennis Center.
"The USTA is obligated to adhere to government guidance at the State, City and County level," the body said.
A spokesman for New York state's health department said the decision was "in the best interest" of health and safety.
The shock intervention came a day after government officials tried to prevent Adrian Mannarino, who had also been in contact with Paire, from playing his last-32 match.
The match got underway three hours after its scheduled start time, after authorities ruled it could go ahead following discussions with tennis officials that also included lobbying by world No 1 Novak Djokovic.
Mannarino and Mladenovic were two of 11 players put under enhanced safety protocols on Monday after Paire was withdrawn from the US Open the previous day following his positive test.
The players signed an agreement with New York City authorities that allowed them to remain in the tournament under the enhanced restrictions, which involved daily testing and barred them from communal areas in their hotel.
Following her first-round singles win on Monday, Mladenovic said she had been "living in a nightmare" following Paire's positive test result.
Players movements are tightly controlled and everyone onsite is being tested regularly to minimise the risk of infection.
Paire, who later tested negative, exited the tournament in a huff, accusing authorities of operating a "fake bubble."
The players who were in contact with him have been told they won't be allowed to fly out of New York until next week.
Back on the court on Saturday, Russian third seed Daniil Medvedev, Italy's sixth seed Matteo Berrettini and Andrey Rublev, all entered the last 16 with easy three-set wins.
Second seed Dominic Thiem also progressed to round four, overcoming 2014 champion Marin Cilic 6-2, 6-2, 3-6, 6-3.
Canada's 94th-ranked Vasek Pospisil shocked eighth-seeded Spaniard Roberto Bautista Agut and Australia's 21st seed Alex de Minaur beat 11th-seeded Russian Karen Khachanov.
In the women's draw, unranked Bulgarian Tsvetana Pironkova, playing her first tournament in three years, knocked out 18th seed Donna Vekic.
Second seed Sofia Kenin defeated Tunisia's Ons Jabeur 7-6, 6-3 to set up a match against Belgium's Elise Mertens for a quarter-final place.
And former No 1 Victoria Azarenka said she had "a great opportunity" to end her seven-year wait for a third Grand Slam after booking a last 16 berth with a straight-sets win over Poland's Iga Swiatek.
