Novak Djokovic survived a mighty scare before sinking wild card Jenson Brooksby in their US Open fourth round clash and staying on course for an historic calendar grand slam.

The world number one was blitzed 6-1 in an astonishing first set by the 20-year-old American, who produced just a single unforced error as he served out nervelessly to take the opener in just 29 minutes.

It took Djokovic until midway through the second set to stem his opponent's remarkable momentum in front of an increasingly raucous home crowd on Arthur Ashe court.

And after ultimately wrapping up a 1-6 6-3 6-2 6-2 victory in three hours, Djokovic admitted: "He just played a perfect first set - I couldn't do much because I was still finding my groove."

If there was an air of inevitability about Djokovic's eventual recovery, the performance of Brooksby, who was ranked outside the world's top 200 four months ago and had never previously faced a top-10 player, was wholly unexpected.

Playing with power and almost perfect depth, he blew his illustrious opponent off the court in the opener and show no signs of ailing in the second, despite Djokovic releasing a huge surge of emotion after breaking to go 2-0 in front.

Brooksby simply responded by converting his sixth break point chance in a fifth game of the set that lasted almost half an hour alone, although Djokovic's ability to immediately re-establish his advantage ultimately proved to be the turning point.

"I think the momentum changed midway through the second set when I came through a couple of long games then I started hitting more cleanly," said Djokovic.

"All in all it was a very physical battle with a lot of exciting rallies, and credit to him for his performance tonight, it was fantastic."

In the quarter-finals Djokovic will meet sixth seed Matteo Berrettini, whom he beat in this year's Wimbledon final, after the Italian's four-set win over Oscar Otte.

Fourth seed Alexander Zverev, a potential semi-final opponent for the Serbian, extended his winning streak to 15 matches with a straight-sets victory over Jannik Sinner.

At 20, Sinner has been usurped as the young gun of men's tennis by Carlos Alcaraz but great things are expected of the Italian and he tested Zverev without being able to take his chances.

Sinner fought back from a break down in the third set and held set point in the tie-break only to miss a routine forehand, and he paid a heavy price, with Zverev triumphing 6-4 6-4 7-6 (7).

The German is riding a wave of confidence, and he said: "I think definitely it's the reason why maybe the big shots in the important moments they seem more natural, I would say, and you're not maybe thinking about it twice like you would in some other situations.

"I'm happy where I am, I'm happy with how things are, and I'm happy with how things were the last few months. I'm in the quarter-finals now and from here on, the matches will definitely not get easier."

Zverev has faced public allegations of domestic violence from an ex-girlfriend - which he denies - as well as a legal battle with his former agent over the past year.

He cited changes in his off-court life as a major influence in his success on it, saying: "The most important decision in my life was I think taking control of my own life.

"That's what I have done at the beginning of this year. I'm taking control over everything - of my life, of my home in Monaco, of my business life, of my management, of companies that I want to work with."

In the next round, Zverev will take on one of the tournament's many surprise packages in South African Lloyd Harris, who reached a first grand slam quarter-final with a 6-7 (6) 6-4 6-1 6-3 victory over 22nd seed Reilly Opelka.