World No 1 Novak Djokovic was dumped out of the Rio Games in the first round on Sunday by Argentina's Juan Martin del Potro, a shock result that reduced the Serb to tears as he left the court.
Djokovic, who won this year's Australian and French Open titles, was the favourite to add Olympic gold to his 12 Grand Slam titles, and Del Potro was as surprised as anyone to see the Serb go out.
"I didn't expect to beat Novak tonight," said del Potro, who has been battling a wrist injury for much of the season.
"But I played an amazing match, my forehand worked great."
Del Potro used his lethal forehand to stun the Serb with a powerful display to win 7-6(4), 7-6(2) in the biggest upset of the tennis tournament.
Earlier in the day, del Potro briefly got stuck in a malfunctioning lift but his luck improved against Djokovic, whom he also defeated in the bronze medal match at the London Games.
Del Potro has fallen outside the top 100 in the world rankings but showed flashes of the hard-hitting brilliance that helped him win the US Open in 2009.
Nadal makes winning return
Spain's Rafael Nadal made a winning return to competition on Sunday at the Rio Games but the former Olympic champion conceded he was not fully fit after being sidelined over two months with a wrist injury.
Third seed Nadal was pumped up for his return and easily overpowered world number 43 Federico Delbonis of Argentina 6-2, 6-1 in the first round but the 14-time Grand Slam winner admitted he was not at his best.
"The wrist needs little bit more time to be 100 per cent recovered," said Nadal, who withdrew from the French Open in May with a wrist injury and then missed Wimbledon. "If there is not Olympic Games here I would not be here competing."
Nadal, the 2008 gold medalist who only confirmed his Rio participation last week, was supported throughout the one-sided match by a rowdy Brazilian crowd.
The 30-year-old Spaniard, who missed a chance to defend his Olympic title with a knee injury, said the pain of missing the 2012 London Games drove him to return to competition sooner than expected.
"It was tough for me to not compete in London, it was one of the toughest decisions in my career. I was close to not being here," said Nadal, who was Spain's flag bearer during Friday's opening ceremony.
"The feeling holding the Spanish flag is something that will stay in my mind for the rest of my life. It was an unforgettable thing for me."