Top seed Novak Djokovic was beaten in the first round of the Olympic men's singles by Juan Martin Del Potro.
The Serb, who lost to Del Potro in the bronze medal play-off at London 2012, was again overpowered by the Argentine, who won 7-6 (7-4) 7-6 (7-2).
The world number one, 29, was in tears as he left the court after the two-and-a-half hours match.
"This is one of the toughest losses in my career," said Djokovic, who was chasing a first Olympic title.
Djokovic became the first man for 47 years to hold all four Grand Slam titles after winning the French Open in June.