RIO: Olympic champion and world number one, Serena Williams was bundled out of the Rio de Janeiro tennis tournament in the third round of the singles on Tuesday after a shock 6-4 6-3 defeat to Ukraine's Elina Svitolina.

The defeat by the 20th-ranked Svitolina ends Williams' Olympic tournament, having also suffered a surprise loss in the doubles with older sister Venus on Sunday.

The 34-year-old Williams, who is not entered in the mixed doubles, cut short her media commitments after the loss, speaking only to the official broadcasters before she returned to the changing rooms.

Svitolina, who will face Czech Republic's Petra Kvitova in the quarter-finals, was over the moon with her first victory in five meetings over the 22-times grand slam singles winner.

“It's an unreal feeling for me. This is just a perfect moment,” said Svitolina.

Williams grew increasingly frustrated during the match as she struggled to find her range, making 37 unforced errors compared to 17 by the 21-year-old Ukrainian.

Late in the second set, Williams served five double faults in a single game, which drew gasps from a shocked audience.

She took out her frustrations on her racquet, slamming it into the court surface on at least two occasions. Williams' defeat came hours after third seed and French Open champion Garbine Muguruza was upset by Puerto Rico's Monica Puig.

Germany's world number two Angelique Kerber, who beat Williams in the final of the Australian Open in January but lost the Wimbledon final to the American, is now the favourite for gold.

Williams had won three doubles Olympic titles with Venus. Their first round loss to Czech duo Barbora Strycova and Lucie Safarova was their first Olympic loss in the doubles.

They won the title in Sydney, did not pair up for Athens due to an injury to Serena but reclaimed the title in Beijing then defended it in London.

They have also won 14 women's doubles grand slam titles.