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 match, which lasted two and a half hours.
"This is one of the toughest losses in my career," said Djokovic, who was trying to win his first Olympic title.