Serena Williams battled into the US Open last 16 Saturday as the tournament endured fresh controversy after New York health officials ordered the women's top doubles pairing be withdrawn over coronavirus restrictions.

Williams, chasing a record-equalling 24th Grand Slam title, 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.

With spectators barred from this year's Flushing Meadows event due to the pandemic, second and fourth seeds Sofia Kenin and Naomi Osaka took advantage of suites they have been assigned to check out the competition.

"It's cool us players can watch each other. It's interesting, I like it," 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, number one seeds Kristina Mladenovic and Timea Babos were yanked 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.

- 'Brutal' -
A spokesman for New York state's health department said the decision was "in the best interest" of health and safety.

But France's Alize Cornet described the move as "very brutal."

"It's definitely weird because she could play her first round of singles and first round of doubles, and now suddenly she's kicked out," Cornet added, after progressing to the last 16 when Madison Keys retired injured.

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 health department contacted the USTA to say Mannarino should be quarantining in his hotel room.

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 number one Novak Djokovic.

Mannarino and Mladenovic were two of 11, mostly French players, put under enhanced safety protocols on Monday after Paire was withdrawn from the US Open the previous day following his positive test.

- Pironkova wins again -
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.

Mannarino said Friday that organizers told him that New York state authorities had taken over the case and ruled that he was now not allowed to play.

Following her first-round singles win on Monday, Mladenovic said she had been "living in a nightmare" following Paire's positive test result.

The US Open is taking place behind closed doors because of the deadly pandemic.

Players movements are tightly controlled and everyone onsite is being tested regularly to minimize 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 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.

And Canada's 94th-ranked Vasek Pospisil shocked eighth-seeded Spaniard Roberto Bautista Agut and Australia's 21st seed Alex de Minaur upset 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 (7/4), 6-3 to set up a match against Belgium's Elise Mertens for a quarter-final place.

And former number one 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.

Second seed Dominic Thiem said he must raise his game if he is to win a first Grand Slam at the 2020 US Open after ousting Marin Cilic in four sets to move into the last 16 Saturday. The 27-year-old Austrian, the losing finalist in the Australian Open this year, battled to a 6-2, 6-2, 3-6, 6-3 victory in the late match at an empty Arthur Ashe stadium.

"I felt a little bit flat," said Thiem, explaining his labors in sets three and four. "It's normal, it's really late, (and) without the crowd," he added.

Thiem said he had yet to play at 100 percent in the tournament.

"I still have to raise my level if I want to go deeper," he explained.

The world number three will play 20-year-old Canadian hotshot Felix Auger-Aliassime for a place in the quarter-finals.

The 15th seed blew away France's Corentin Moutet 6-1, 6-0, 6-4 in just 1hr 57 mins, becoming the first player born in the 2000s to reached a Grand Slam fourth round.

"Dominic has been getting stronger and stronger year after year," said Auger-Aliassime.

"You feel like step by step he's been reaching later stages of these tournaments, really becoming like a power in the tennis world. For me, it would be a good matchup," he added.

Auger-Aliassime was joined in round four by 94th-ranked compatriot Vasek Pospisil, who shocked eighth-seeded Spaniard Roberto Bautista Agut, 7-5, 2-6, 4-6, 6-3, 6-2.

Pospisil, 30, struck 70 winners to reach the last 16 at Flushing Meadows for the first time.

"I'm thrilled. I was playing great tennis all the way through," he said.

With 12th seed Denis Shapovalov already through, the next round marks the first time three Canadian players have reached the second week of a Grand Slam at the same time.

Pospisil will take on 21st seed Alex de Minaur for a place in the last eight after the Australian rallied from two-sets-to-one down to upset 11th-seeded Russian Karen Khachanov 6-4, 0-6, 4-6, 6-3, 6-1.

The Aussie number one broke serve six times to set up the match with Pospisil, who he has beaten in two career ATP meetings.

"Definitely building some confidence, and hopefully I can keep this momentum going," de Minaur said.

- 'Businesslike' -
Elsewhere, America's Frances Tiafoe obliterated Hungarian Marton Fucsovics 6-2, 6-3, 6-2 in 1h 44m to set up a last 16 showdown with Russian third seed Daniil Medvedev.

"It was definitely one of my best performances start to finish," said Taifoe, the last remaining American in the men's singles.

"I wasn't trying to be too fancy. I just kind of got it done today. Very professional, very businesslike," added the 22-year-old.

Italian sixth seed Matteo Berrettini and Russia's 10th seed Andrey Rublev both enjoyed easy three-set wins to set up a last-16 clash against each other.

Rublev only gave up four games as he pulverised unseeded Italian Salvatore Caruso 6-0, 6-4, 6-0.

"It's going to be really tough. He's serving well. Forehand and backhand, they are great, like, weapons," Berrettini said of Rublev.

Top seed and world number one Novak Djokovic is overwhelming favorite to win an 18th Grand Slam at the 2020 US Open.