Defending US Open champion Novak Djokovic suffered a shock exit at Flushing Meadows as he lost in four sets to Alexei Popyrin. 

Djokovic arrived at the tournament with confidence following his gold medal triumph at the Paris Olympics, and was tipped to lift a record-extending 25th grand slam title.

However, the Serbian's hopes of a fifth success in New York were dashed as he followed fellow favourite Carlos Alcaraz out of the competition in the third round. 

Popyrin emerged victorious in three hours and 18 minutes, prevailing 6-4 6-4 2-6 6-4 to confirm Djokovic's earliest exit from the tournament in 18 years.

"I have played some of the worst tennis I have ever played," Djokovic said.

"Honestly, the way I felt and the way I played from the beginning of this tournament, third round is a success."

"I tried my best, I didn't have any physical issues. I just felt out of gas, and you could see that. From the first match, I just didn't find myself on this court.

"Life moves on. I'll try to recalibrate and look forward to what's next."

Data Debrief: Another one bites the dust

Djokovic was far from his clinical best, serving a career-high 14 double faults in a grand slam match and committing 49 unforced errors.

The Serbian will also end the year without winning a grand slam title, the first time he has done so since 2017.

But the day belonged to Popyrin, saving 12 of the 16 break points he faced against the world number two, finding particular success with his forehand as 31 of his 49 winners came from that shot.