Popyrin enjoyed a positive build-up to the US Open, winning his first ATP Masters 1000 title earlier this month in Montreal.

The Sydney-born player beamed with confidence as he battled through the first set, holding off five break points before striking at 4-4 and serving out.

In the second set he looked even better, steadily working his serve and winning 88% of the points on his first serve, while Djokovic found himself in more and more trouble.

The 24-time major winner finally came to life early in the third set, winning the first three games before a slump allowed Popyrin to break back.

But Djokovic responded quickly, adding to the noise from the enthusiastic crowd at Arthur Ashe Stadium, restoring his lead and breaking for the third time to keep his title defence alive.

Djokovic was looking to win his fifth US Open, which would see him surpass Margaret Court and set the record for most Grand Slam singles titles.

The two remained evenly matched in the fourth set, but Popyrin put Djokovic in trouble when he successfully converted his fourth break point in the fifth game.

He held his ground in his next service game, clearing two break points before a series of double faults and sloppy errors from the usually clinical Djokovic brought the No. 28 to the brink of a landslide victory.

Popyrin was double-broken and his first attempt to serve out the match failed. However, he was more successful the second time around and served three times with great force before Djokovic got the first match point far over the line.

“It’s unbelievable because I’ve reached the third round about 15 times in my career, but never the fourth round,” Popyrin said.

“To be able to compete against the greatest boxers of all time and make it to the fourth round is unbelievable. It’s a great feeling that the hard work has paid off.”