TENNIS: The second day at the Olympics tournament was a day of shocks, with Novak Djokovic bowing out in the first round of the singles before the Williams sisters lost for the first time in Games history in the doubles.

Djokovic lost 7-6 (7-4) 7-6 (7-2) to Juan Martin del Potro, who also beat him in the bronze medal match at the London Olympics.

Venus and Serena Williams had won 15 out of 15 matches at the Olympics, winning gold in 2000, 2008 and 2012.

They lost to Czech Republic's Lucie Safarova and Barbora Strycova 6-3 6-4, who had only played one match together before.

Rafael Nadal and Andy Murray were amongst the other winners on Sunday.

Serena did have better luck in the singles, the defending champion Serena Williams too strong for Australian Daria Gavrilova in the first round.

Williams had to fight hard to win her opening service game, was broken on her next serve and then again when trying to win the first set at 5-3.

But the American broke straight back to take the first set 6-4 and eased through the second 6-2.

Gavrilova struggled in the second set after playing and losing her doubles clash with Sam Stosur the previous day to Switzerland's Martina Hingis and Timea Bacsinszky.

The Russian-born 22-year-old admitted she got tired against Williams.

"I didn't have enough rest as I played pretty late and I think it affected me,” she said.

"You can't be flat playing against Serena because you have to run all the time.”

"It's not the result I wanted but here nothing counts unless you get a medal.

"We tried yesterday in doubles and I gave my all today, no regrets.”

Sam Groth, a late call-up for the singles, lost in round one to Belgian eighth seed David Goffin 6-4 6-2.

There was better news for Samantha Stosur, who fought back from a set down to defeat Jalena Ostapenko of Latvia 1-6 6-3 6-2 to make it through to round two.