RIO DE JANEIRO, Aug 9 (Reuters) - Olympic champion and world number one Serena Williams tumbled out of the Rio de Janeiro singles competition in the third round on Tuesday after a shock defeat to Ukraine's Elina Svitolina.

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Svitolina, who is ranked 20th in the world, stunned the London Games gold medallist 6-4 6-3 to record a memorable victory against the American, who has won 22 grand slam singles titles.

"It's an unreal feeling for me. This is just a perfect moment," said Svitolina, who will face Czech Republic's Petra Kvitova in the quarter finals.

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(Photo by Mike Ehrmann/Getty Images) RIO DE JANEIRO, BRAZIL - AUGUST 06: Bret Bonanni #10 of United States shoots the ball against Croatia during the Men's Water Polo Preliminary Round - Group B match between the United States of America and Croatia at the Maria Lenk Aquatics Centre on August 6, 2016 in Rio de Janeiro, Brazil. (Photo by Tom Pennington/Getty Images) RIO DE JANEIRO, BRAZIL - AUGUST 07: Laura Giombini of Italy (R) reacts during the Women's Beach Volleyball preliminary round Pool D match against Jamie Lynn Broder and Kristina Valjas of Canada on Day 2 of the Rio 2016 Olympic Games at the Beach Volleyball Arena on August 7, 2016 in Rio de Janeiro, Brazil. (Photo by Shaun Botterill/Getty Images) RIO DE JANEIRO, BRAZIL - AUGUST 07: Robert Gardos of Austria plays a Men's Singles second round match against Ovidiu Ionescu of Romania on Day 2 of the Rio 2016 Olympic Games at Riocentro - Pavilion 3 on August 7, 2016 in Rio de Janeiro, Brazil. 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Williams was visibly frustrated throughout the match as she struggling to find her range, making 37 unforced errors compared to 17 by Svitolina.

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

Williams, who has won three women's doubles Olympic titles with sister Venus, was also eliminated with her sister from the doubles on Sunday.

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(Reporting by Drazen Jorgic and Alexandra Ulmer; Editing by Greg Stutchbury)