The Serbian, 37, beat Radu Albot in straight sets in his first match and was 4-6 4-6 0-2 up when opponent Laslo Djere withdrew on Wednesday. And he branded his display against Popyrin “awful” after playing “some of the worst” tennis of his glittering career.

"Congratulations to him and his team," Djokovic said. "He definitely played better and deserved to win today.

"I mean, on my end, honestly, the way I felt and the way I played from the beginning of this tournament, third round is a success. I mean, I have played some of the worst tennis I have ever played, honestly, serving by far the worst ever.

"So, yeah, if you play on a quick surface like this without, you know, the serve, without ability to win free points there, very low first-serve percentage, many double faults, then you can't win.

"You can't win, especially against the guys who are in form like Alexei who is serving big, puts a lot of pressure on your service game. Yeah, it was just an awful match for me."

Asked why his level dropped so dramatically since his incredible performance in the Olympic gold medal match, he continued: "I don't know. Maybe, maybe. But it's a different surface.

"I mean, obviously it had an effect. 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 US Open, you know, 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, you know, and you could see that with the way I played.

"Just from the very beginning, from the first match, I just didn't find myself at all on this court. It's all I can say, you know. You know, life moves on. I mean, yeah, just try and recalibrate and look for what's next."