NEW YORK — Novak Djokovic did not go easy on himself when assessing the listless way he performed from the start of the U.S. Open, pointing to his sloppy serving as the main reason the defense of his 2023 title surprisingly ended in the third round.

“I have played some of the worst tennis I have ever played, honestly,” Djokovic said, just after midnight as Friday turned to Saturday. “Serving — by far — the worst ever.”

With 14 double-faults, raising his tournament total to 32, Djokovic bowed out with a 6-4, 6-4, 2-6, 6-4 loss to 28th-seeded Alexei Popyrin of Australia, another shocking result at Flushing Meadows one night after Carlos Alcaraz exited.

“It was just an awful match for me,” the No. 2-seeded Djokovic said. “I wasn’t playing even close to my best. It’s not good to be in that kind of state where you feel OK physically, and of course you’re motivated because it’s a Grand Slam, but you just are not able to find your game. That’s it. The game is falling apart, and I guess you have to accept that tournaments like this happen.”

Not often for him, though.

After all, Djokovic was trying to become the first player in tennis history with 25 Grand Slam singles titles. Instead, after knee surgery in June, he finishes a year without claiming at least one major championship for the first time since 2017. Before that, it hadn’t happened since 2010.

Alexei Popyrin of Australia reacts against Novak Djokovic during a third round match of the U.S. Open tennis championships on Friday.

Julia Nikhinson/AP

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Julia Nikhinson/AP

Also of note: 2024 now becomes the first season since 2002 in which none of the Big Three of men’s tennis — Djokovic, Rafael Nadal and Roger Federer — earned a Slam trophy.

The third-round exit equals Djokovic’s worst showing at Flushing Meadows; the only other occasions he was beaten that early at the U.S. Open came in 2005 and 2006. The man who defeated Djokovic 18 years ago, International Tennis Hall of Fame member Lleyton Hewitt, is now Australia’s Davis Cup captain and was sitting in Popyrin’s guest box in Arthur Ashe Stadium.

Djokovic, who is 37, has reached the final in Ashe 10 times, leaving with the title in 2011, 2015, 2018 and 2023.

On Friday, though, he was sluggish and emotionally flat, perhaps residual fatigue after collecting his first Olympic gold medal for Serbia by beating Alcaraz in the final at the Paris Games earlier in August.

“Obviously, it had an effect,” Djokovic said. “I spent a lot of energy winning the gold, and I did arrive to New York just not feeling fresh mentally and physically. But because it’s the U.S. Open, I gave it a shot and I tried my best. I mean, I didn’t have any physical issues. I just felt out of gas. And you could see that with the way I played.”

The No. 3-seeded Alcaraz entered the U.S. Open as the tournament favorite having won the French Open and Wimbledon, and acknowledged his energy was lower than he realized after getting eliminated by 74th-ranked Botic van de Zandschulp 6-1, 7-5, 6-4 on Thursday night.

Djokovic then replaced Alcaraz as the money-line pick to take the men’s title, according to BetMGM Sportsbook, but that status didn’t last long at all.

For the 25-year-old Popyrin, this represents a real breakthrough: He had been 0-3 against Djokovic and 0-6 in third-round matches at majors.

But the strong-serving Popyrin is playing as well as ever, coming off the biggest title of his career less than three weeks ago at a hard-court tournament in Montreal, where he picked up five wins against opponents ranked in the top 20.

Everything was working against Djokovic.

Popyrin was terrific at the net, going 10 for 10 on serve-and-volley approaches and 25 for 36 overall on points when he pushed forward. Djokovic, in contrast, only won the point on 19 of his 40 trips to the net, in part because Popyrin kept flipping passing shots by him.

Popyrin took big cuts with his powerful forehand, accumulating 22 of his 50 total winners with that shot.

And he broke Djokovic five times, including for a lead of 3-2 in the fourth. That game felt titanic, lasting more than 10 minutes and including four break chances for Popyrin, who converted the last with an inside-out forehand to close a 22-stroke exchange, then rocked back on his heels, clenched both fists and let out a roar. He took Djokovic’s next service game, too, to make it 5-2.

The first time Popyrin served for the match, he faltered, allowing Djokovic to break. The second time, Popyrin finished the deal, holding at love when Djokovic sent a forehand long.

Now Popyrin will try to reach his first Grand Slam quarterfinal by getting past No. 20 Frances Tiafoe, who advanced Friday with a 4-6, 7-5, 6-7 (5), 6-4, 6-3 win over No. 13 Ben Shelton in a matchup between two Americans.

“If he serves well, plays well, he can beat anybody,” Djokovic said about Popyrin. “Look, Alcaraz is out. I’m out. Some big upsets. The draw is opening up.”