NEW YORK — A US Open match that Novak Djokovic already expected to be difficult was threatening to become a toss-up as he struggled with his serve.

Missing more than he made, Djokovic couldn’t get many quick points Wednesday night. He acknowledged that he should have lost the second set, and if his opponent, Laslo Djere, didn’t get injured and have to stop, maybe Djokovic wouldn’t even have won that match.

Once he had, however, the defending champion was only looking ahead.

“The matches are only going to get tougher from here. I know that, but I’m fine,” Djokovic said. “I’ll find my way, as I have done many times in my career.”

Djokovic reached the third round when Djere had to retire with the No. 2 seed leading his Serbian countryman 6-4, 6-4, 2-0.

Djere was leading 4-2 in the second set when he appeared to be troubled by pain near his hip and was visited by an athletic trainer later in the set. He finished that set, which lasted 69 minutes after the first one went 60, but he didn’t last much longer.

“In the end, not the kind of finish that we players or crowd wants to see, but I think it’s probably due to that physical battle that we had in the opening two sets,” Djokovic said.

It was the 90th win at the US Open for Djokovic, making him the first man to reach that total at all four Grand Slams. The 24-time major champion will play No. 28 seed Alexei Popyrin on Friday.

It marked the 16th time Djokovic has advanced at a major via retirement or walkover, the most by any man since the Open era began in 1968.

Djere was the only player to win a set against Djokovic at last year’s US Open, grabbing the first two in their third-round encounter before Djokovic rallied.

“So, I knew coming into the match that if I don’t serve well, which was the case, I’m going to have to really grind and work for my points a lot,” Djokovic said. “That’s what I guess caused the two sets to be played over two hours.”

Djokovic made only 47% of his first serves and appeared to be struggling physically himself earlier in the match, which was just his second since winning the gold medal in the Paris Olympics.

Djere had two chances to break Djokovic’s serve for what would have given him a 5-2 lead in the second set. Instead, he wouldn’t win another game, with Djokovic breaking to win the set when Djere yanked a forehand out of bounds.

“All in all, of course, I have to be happy with the win,” Djokovic said, “and happy that in important moments I managed to play one ball more than him over the net, I guess find the right shots or anticipate well as I did in the set point in the second set.”

Information from The Associated Press was used in this report.