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.

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

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