Olympics Rio 2016: Novak Djokovic shocked by "tough" defeat to Juan Martin del Potro

Djokovic, who also lost to Del Potro in the bronze medal play-off at London 2012, had no answer to the power of the Argentinian, with the 2009 US Open champion sealing a 7-6 (7/4) 7-6 (7/2) victory in almost two and a half hours.

The 27-year-old, who has dropped to 141st in the world after almost two years out with wrist problems, did not face a single break point on his serve and hammered 29 forehand winners past a helpless Djokovic.

An Olympic gold medal is the biggest prize missing from the Serbian's trophy cabinet and the wait goes on until at least Tokyo 2020 following his stunning setback.

"No doubt this is one of the toughest losses in my life and in my career," he said. "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."

Djokovic's winning grand slam run came to an end with a shock third-round loss to Sam Querrey at Wimbledon, but the 29-year-old had bounced back by claiming his 30th Masters title in Toronto last week and was a hot favourite in Rio, only to bow out at the first hurdle.

He said: "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."

Djokovic had come out on top in his last three clashes against Del Potro, most notably the 2013 Wimbledon semi-final in a magical five-set encounter which spanned four hours and 43 minutes.

Despite their last meeting being later that year and Del Potro only recently starting his latest comeback from a lengthy injury lay-off, there was still some expectation that this would be the pick of the ties in the opening round.

And Del Potro certainly delivered as he claimed only his fourth win in 15 meetings against his illustrious opponent - although even he thought victory would be beyond him.

He said: "It was a wonderful evening from the beginning. Since the draw, the anticipation was very high and I believe I did what I planned for the match. I didn't expect to beat him. I'm surprised with the level I showed. After all the effort I've put in to get back to playing tennis, I've defeated the number one.

"I felt once again I had the capability to play against the best in the world. After everything I've been through with my wrist. I also played well with my forehand. When I hit, the crowd was cheering from the stands. I love that. It gives me more motivation. It was a dream night."