Serbia's Novak Djokovic has lost his second match in as many days at the Rio Olympics after he and Nenad Zimonjic exited the men's doubles in round two. The world number one and his 40-year-old team-mate lost 6-4 6-4 to Brazilians Marcelo Melo and Bruno Soares.

The loss came 24 hours after the 29-year-old star was defeated by Argentine Juan Martin del Potro in the men's singles competition and left the court in tears. The Serb, who lost to Del Potro in the bronze-medal play-off in London four years ago, was again beaten 7-6 (7-4) 7-6 (7-2).

That heart-breaking defeat was one of the toughest of Djokovic's career. "It's not easy to handle, especially now, just after the wounds are still fresh," he said, according to the BBC.

Djokovic's Olympic disappointment marks the end of a particularly bad run for the world number one, who also suffered a surprise third-round defeat at Wimbledon in July to big-serving American Sam Querrey.

Elsewhere in Rio, Britain's Johanna Konta reached the third round of the women's singles, but teammates Heather Watson and Kyle Edmund have been eliminated from the competition. Konta, the world number 13, beat France's Caroline Garcia 6-2 6-3 and subsequently said she was relishing her Olympic experience.

"I'm grateful to be here and I get to say I'm an Olympian," she told BBC Radio 5 live. "I'm here to compete and I'm really enjoying being part of the whole thing." Next, Konta will play Russia's Svetlana Kuznetsova in the last 16 of the singles.

Meanwhile, Watson was beaten 6-3 1-6 6-3 by Elina Svitolina of Ukraine, while Edmund exited the men's singles with a 6-4 7-5 loss to Taro Daniel of Japan, who is ranked 117 in the world.