Juan Martin del Potro stunned a tearful Novak Djokovic in an emotional Olympics triumph on Sunday as Venus Williams pondered the end of her four-gold Games odyssey.

Del Potro, the 2009 US Open champion who was pushed to the brink of retirement by a series of wrist injuries, downed the top seed 7-6 (7/4), 7-6 (7/2).

The giant Argentine had also defeated the 12-time major winner in the bronze medal match at the London Olympics in 2012.

Both players embraced at the net and wept at the end of their centre court duel played out in front of a raucous Argentine and Serb crowd.

Del Potro, now ranked at 145 in the world after being number four in 2010, said Sunday's win was more special than his victory in 2012.

"This victory is bigger than last time because I know how tough it was to come back from three surgeries," said the 27-year-old who unleashed more than 40 winners.

"Tonight I played one of the best matches of my career."

"Delpo was the better player and he deserved to win. That's sport," said world number one Djokovic, whose Olympic singles record is stalled at a bronze from Beijing in 2008.

"It's very sad and disappointing to go out of the tournament this early but I am glad that a good friend of mine, who has struggled with injuries, has won."

Meanwhile, four-time gold medallist Venus Williams was staring at the end of her 16-year, five-Olympics journey.

Williams, 36, and sister Serena suffered their first ever defeat in women's doubles, a record which boasted 15 successive match wins and golds in Sydney in 2000, Beijing in 2008 and London four years ago.

The 6-3, 6-4 loss to Czech pair Lucie Safarova and Barbora Strycova came just a day after Venus had been knocked out of the singles in what was her first opening round Olympics loss.