Novak Djokovic has described his first-round exit at the Rio Olympics as one of the "toughest losses" of his career.

The world number one was the favourite to claim gold at the Olympic Tennis Centre, but crashed out at the first hurdle courtesy of a straight-sets defeat at the hands of Juan Martin del Potro.

The Serbian left the court in tears following the defeat, and he made no secret of his disappointment at once again missing out on the chance to win Olympic gold.

"No doubt this is one of the toughest losses in my life and in my career. It's not easy to handle, especially now, just after the wounds are still fresh. But, you know, you have to deal with it. It's not the first or the last time that I have lost a tennis match. But the Olympic Games, yeah, it's completely different," he told reporters.

"Delpo was the better player and he just deserved to win. That's sport. He just came up with some extraordinary tennis and I have to congratulate him.

"I'm very sad and disappointed from my side to go out in a tournament this early, but on the other hand I'm glad a good friend of mine, and somebody who has struggled the last couple of years with injuries, is back and playing at this level."

Del Potro also beat Djokovic in the bronze-medal match at the London Olympics four years ago.