NEW YORK (AFP) — Defending champion Novak Djokovic admitted he had played “his worst tennis ever” as he slumped to his earliest exit at the US Open in 18 years on Friday, sparking more questions over his long term future in the sport.

The 37-year-old Djokovic, a four-time champion in New York, lost 6-4, 6-4, 2-6, 6-4 in the third round to 28th-ranked Alexei Popyrin of Australia, leaving him frustratingly short once again of winning a record 25th Grand Slam title.

His defeat, which saw him hit an uncharacteristic 14 double faults and commit 49 unforced errors, means that for the first time since 2017 he will end the season without a Grand Slam title.

“The way I played from the beginning of the tournament reaching the third round is a success,” said a despondent Djokovic, who remains marooned on 24 Slam titles with Margaret Court.

“I have played some of my worst tennis ever.”

Djokovic admitted that the effort and aftermath of winning Olympic gold in Paris just four weeks ago impacted his lackluster New York campaign.

“Obviously, it had an effect. I spent a lot of energy winning the gold, and I did arrive in New York just not feeling fresh mentally and physically.”

“But because it’s the US Open, I gave it a shot and I tried my best. I didn’t have any physical issues. I just ran out of gas.”

The latest crushing setback comes in a year which saw Jannik Sinner succeed him as Australian Open champion before Carlos Alcaraz took his French Open crown and swept him off court in the Wimbledon final.

Having defeated him in the Australian Open semi-finals, Sinner went on to also take his world number one ranking which Djokovic had held for a combined 428 weeks.

A knee injury, which required surgery, forced Djokovic to withdraw from the French Open on the eve of his quarterfinal.

His recovery still wasn’t strong enough to prevent Alcaraz from claiming an eighth Wimbledon.

The stunning result means that for the first time since 2002, none of the Slams will have been won by any members of the “Big Three” of Djokovic, the absent Rafael Nadal or the retired Roger Federer.

The Serbian’s exit on Friday was another blow to the US Open after Alcaraz was shocked in the second round on Thursday.

“I played some good tennis,” said Popyrin, who was watched in his player’s box by Lleyton Hewitt, the man who stunned Djokovic in the third round in New York in 2006.

“To be in the fourth round of a Grand Slam by beating the greatest of all time is unbelievable.”

The only consolation for Djokovic in a frustrating year was winning Olympic gold in Paris earlier in August.