In a night of huge upsets at the Olympics, world number one Novak Djokovic was knocked out in the tennis first round by Juan Martin Del Potro, and the doubles team of Serena and Venus Williams also crashed out in spectacular fashion.
Del Potro, ranked 145 in the world, stunned Djokovic late on Sunday with a powerful display to win 7-6, 7-6.
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The Serb was left standing in tears in front of a raucous crowd in Rio de Janeiro, as the injury-plagued Argentinian put on one of his best performances in years.
"Delpo was the better player and he deserved to win," Djokovic was quoted as saying by the Associated Press news agency.
"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."
Del Potro, a former US Open champion, made his first Grand Slam appearance in two and half years last month, beating fourth-seeded Stan Wawrinka at Wimbledon.
"Tonight I played one of the best matches of my career," Del Potro said, as he prepared to face Joao Sousa, ranked 36, in the second round.
The Williams sisters crashed out to the Czech Fed Cup winners Lucie Safarova and Barbora Strycova, losing 6-3, 6-4 in what Serena called a terrible performance.
"We played terrible and that basically showed in the results," Serena said. "I don't know why. I wasn't playing the way I needed to play, I wasn't crossing the way I needed to cross. It was what it was."
Venus Williams was not at full strength as she was suffering from a virus. The 36-year-old also crashed out in the first round of the singles, but on Sunday said she did not want to use that as an excuse against the Fed Cup champions.
Wimbledon champion Andy Murray and doubles partner, his brother Jamie, also went out 7-6, 7-6 against Brazilians Thomaz Bellucci and Andre Sa.