Novak Djokovic said he "served awful" after progressing to the third round of the US Open.

Djokovic was being pushed hard by his Serbian compatriot Laslo Djere in Wednesday's headline clash at Flushing Meadows.

Indeed, the reigning champion needed two hours to take the first two sets, but Djere was eventually forced to retire through injury, handing Djokovic a 6-4 6-4 2-0 victory.

"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 victory brought up Djokovic's 90th match win at the US Open, moving him one clear of Roger Federer, with only Jimmy Connors (98) ahead of the 37-year-old.

"It doesn't get bigger than this in terms of a tennis stage," said Djokovic, who will face Australia's Alexei Popyrin in the next round.

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

Data Debrief: Djokovic stands alone

Djokovic is now the only male player in the Open Era to win at least 90 matches at each of the four grand slams.

This win also improved Djokovic's career record against Djere to 3-0. His overall record against fellow Serbians, meanwhile, stands at 30-5, while he has only lost once in 19 hard-court matches against his compatriots, going down to Janko Tipsarevic at the ATP Finals in 2011.