Novak Djokovic sailed into the last 16 of the US Open on Friday as off-court intrigue forced the delay of a men's singles match over issues linked to coronavirus.

Djokovic outclassed Germany's Jan-Lennard Struff 6-3, 6-3, 6-1 in a routine victory for the world number one, who is chasing an 18th Grand Slam singles title.

Far from routine was a last-minute attempt by New York health officials to prevent France's Adrian Mannarino from playing his last-32 encounter against German fifth seed Alexander Zverev.

The intervention led to a back and forth between tournament organizers and the state government and resulted in the match, which Zverev won in four sets, starting over three hours late.

"It was a weird situation for me," said Mannarino, one of 11 players put under enhanced safety protocols earlier this week after compatriot Benoit Paire was withdrawn from the tournament after testing positive for coronavirus.

"I was just laying on the sofa still trying to be focused just in case I would go on court," he said, following Zverev's 6-7 (4/7), 6-4, 6-2 victory.

Zverev will now play unseeded Spaniard Alejandro Davidovich Fokina in the last 16 while Djokovic will play Spain's Pablo Carreno Busta for a place in the quarter-finals.

- Osaka struggles -

Earlier, Japan's Naomi Osaka battled into the last 16 of the women's draw.

The two-time Grand Slam winner required three sets and 2hr 33 min to dislodge 18-year-old Ukrainian Marta Kostyuk in an early game at Arthur Ashe Stadium.

The Japanese fourth seed, who threw her racquet in frustration after losing the second set on tie-break, prevailed 6-3, 6-7 (4/7), 6-2 against the world number 137.

"I just felt like I had so many points that I didn't capitalise on. I think when I went up, I became a bit passive and then she came in because she has no fear," said Osaka, 22.

Her experience came to the fore in the deciding set as she held her serve and broke Kostyuk twice to set up a match against 14th seed Anett Kontaveit of Estonia for a place in the quarter-finals.

Elsewhere, sixth seed Petra Kvitova breezed past American Jessica Pegula 6-4, 6-3 to set up a last-16 encounter with Shelby Rogers, also of the United States.

Eighth seed Petra Martic of Croatia swept aside unseeded Russian Varvara Gracheva 6-3, 6-3.

Martic will play Yulia Putintseva of Kazakhstan in the last 16 after the 23rd seed powered past Vera Lapko of Belarus 6-3, 6-3.

Germany's 17th seed Angelique Kerber also progressed to the fourth round, making light work of American Ann Li 6-3, 6-4.

Kerber, the 2016 US Open champion, will go up against Jennifer Brady for a quarter-final spot after the American 28th seed knocked out France's Caroline Garcia 6-3, 6-3. 

Garcia had provided the biggest upset of the tournament on Wednesday when she eliminated top seed Karolina Pliskova.

- Tsitsipas stunned -

In the men's draw, Croatia's 27th seed Borna Coric staged a remarkable Houdini act to stun fourth seed Stefanos Tsitsipas in a 4hr 36min late-night thriller.

Coric appeared to be heading for certain defeat after Tsitsipas, leading by two sets to one, opened up a 5-1 lead in the fourth.

But Coric rallied superbly, saving six match points and fighting back to take the set 7-5 to force a decider.

Tsitsipas went a break up once more in the fifth but couldn't press home the advantage as Coric forced a tie break before clinching a 6-7 (2/7), 6-4, 4-6, 7-5, 7-6 (7/4) win.

Twelfth seed Denis Shapovalov recovered from 5-2 down in the fourth set to pip American number 19 seed Taylor Fritz 3-6, 6-3, 4-6, 7-6 (7/5), 6-2 

Fritz, 22 was within two points of victory before the 21-year-old Shapovalov completed a remarkable comeback.

"I think we both deserved to win," said Shapovalov. "It was great tennis and I think I just got a little bit lucky."

The Canadian will do battle with seventh seed David Goffin for a place in the quarter-finals after the Belgian brushed aside Serbia's 26th seed Filip Krajinovic 6-1, 7-6 (7/5), 6-4.

Britain's involvement in the singles ended on Friday with both Dan Evans and Cameron Norrie exiting.

France's Corentin Moutet knocked out Evans 4-6, 6-3, 7-6 (7/5) 7-6 (7/1) as they resumed their second round match that was postponed Thursday because of rain.

Serena headlines day six at the US Open

Serena Williams takes on 2017 US Open winner Sloane Stephens for a place in the last 16 at Flushing Meadows on Saturday.

The third-round match is the drawcard event on day six at the Billie Jean King National Tennis Center in New York.

Stephens dumped Williams out of the 2013 Australian Open during her sensational run to the semi-finals as a 19-year-old.  

"She's a great competitor," Williams said of her fellow American, who is seeded 26th.

"It's an incredibly interesting match, because she's actually a US Open champion. She's a great player. You can't win a Grand Slam and not be really, really, really, really, really good."

Williams, the tournament's third seed, is seeking a 24th singles Grand Slam title that would equal Margaret Court's record.

It has been more than three years since Williams won her 23rd Grand Slam title at the 2017 Australian Open -- when she was already pregnant with daughter Olympia.

The 38-year-old has come close since, reaching four major finals only to come away empty handed.

A tournament victory next week would also see her break away from Chris Evert, on six titles, as the most decorated US Open champion of the women's game in the modern era.

In the men's draw Saturday, second seed Dominic Thiem takes on 2014 US Open champion Marin Cilic for a place in the last 16.

And third seed Daniil Medvedev appears against American wildcard J.J. Wolf in a last-32 match.