World number one Novak Djokovic of Serbia left in tears after his dream of getting the elusive Olympic gold ended in form of a 7-6 (7-4), 7-6 (7-2) defeat against Argentina's Juan Martin del Potro.
The former US Open champion Del Potro clinched the big win on second match point with a little help from the netcord on a forehand stroke after 2 hours 27 minutes.
The two embraced at the net and the tears then flowed as the disappointed Djokovic failed again in his bid for the elusive gold. He completed a career grand slam at the French Open in June but then suffered a surprise defeat in the third round of Wimbledon against Sam Querrey.
"No doubt this is one of the toughest losses in my life and in my career," Djokovic said. "It's not the first or the last time I am losing a tennis match. But the Olympic Games, yeah, it's completely different."
Del Potro meanwhile cried tears of joy after the monumental win, only his fourth in the 15th meeting with Djokovic, but the second one at the Olympics as he also won their bronze medal game at London 2012.
He fired 41 winners to Djokovic's 24 and also had the upper hand in unforced errors, 24-32.
"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," Del Potro said.
"He’s a great champion, a friend for a long time. I know how important this event was for him. He wanted the gold medal, but I also believe he knows about my effort. It was a spectacular match from any point of view."
Del Potro stole the show even though he was stuck in an elevator for 40 minutes in the athletes' village in the morning - in front of a noisy crowd with a big Argentine contingent and the Brazilian fans firmly behind Djokovicv against the player from their regional arch-rivals.
The 2009 US Open winner Del Potro has dropped to 145th in the rankings after several wrist operations but was considered the most dangerous floater in the field and the toughest draw Djokovic could have had.
Djokovic's defeat further opens up the Rio draw that saw only five top-10 players, with Swiss Roger Federer and Stan Wawrinka the most prominent absentees.
Djokovic was in good company on the sidelines though on a windy and cool day, as Serena and Venus Williams' bid for a fourth straight women's doubles also ended in the first round, just as Wimbledon champion Andy Murray's doubles campaign with his brother Jamie.
The Williams sisters lost a first Olympic doubles match when they were ousted 6-3, 6-4 defeat against Czech Fed Cup winners Lucie Safarova and Barbora Strycova. The sister act had won 15 matches in a row en route to gold in 2004, 2008 and 2012, and each also has a singles gold.
Venus Williams was however not at full strength as she is suffering from a virus. The 36-year-old also crashed out in the first round of the singles, but on Sunday didn't want to use that as an excuse against the Fed Cup champions.
"Today was not our best day as a team. We like to play into the tournament but we played a tough team," she said.
Serena agreed: "We played terrible and that basically showed in the results. 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."
The Murrays meanwhile went out 7-6 (8-6) 7-6 (16-14) against Brazilians Thomaz Bellucci and Andre Sa.