Novak Djokovic is through to the third round of the US Open after a gruelling match came to a premature end when Laslo Djere was forced to retire with injury.

Defending champion Djokovic was leading 6-4 6-4 2-0 when an abdominal issue meant fellow Serb Djere was unable to continue.

Although the match went little more than two sets, the pair were on court for more than two hours in extreme heat and humidity in New York.

“It was a big fight, more than two hours for two sets,” Djokovic said.

“I served awful and without the serve, you have to grind, you have to run.”

The two scrapped for an hour in a ferociously contested first set before Djokovic found an extra gear to take it with a break to love.

Djokovic appeared to be struggling in the humid conditions early on and received treatment before the second set.

Djere sensed his chance, earned an early break and looked set to level the match at a set apiece, leading 4-2 and with two break points.

However Djokovic held serve and, with his compatriot suddenly showing signs of discomfort, broke back in the next game.

A medical timeout did little to help Djere as Djokovic broke again to snatch the second set.

Despite being visibly in pain, Djere attempted to play on but when he failed to win a point in the first two games of the third set, he approached the net and retired.

That handed Djokovic his 90th win at Flushing Meadows, making him the only man to have won 90 matches at all four Grand Slams.

“It doesn’t get bigger than this in terms of a tennis stage,” the 37-year-old added.

“The atmosphere is incredible and it’s a great privilege to be able to play here again at my age.”

Djokovic will face 28th seed Alexei Popyrin of Australia in round three.