Top seed and defending champion Williams was clearly struggling with a right shoulder injury as she was stunned 6-4 6-3 by Svitolina, 13 years her junior.

"The better player won," said Williams, who served up eight double faults, including five in one game in the second set. She also hit 37 unforced errors.

"It was a great opportunity. It didn't work out the way I wanted it to, but at least I was able to make it to Rio. That was one of my goals."

Defeat for the 34-year-old means that the Rio tournament has lost both its number one players before the quarter-finals after Novak Djokovic went out in the first round.

Only three of the top 10 seeds have made it to the last-eight of the women's draw. The Rio Games -- and possibly her Olympic career -- are now over for Serena after she and sister Venus were deposed as doubles champions in the first round.

Williams was joined at the exit on Tuesday by third seed and French Open champion Garbine Muguruza who lost to Puerto Rico's Monica Puig. However, there were no such problems for defending men's champion Andy Murray and 2008 winner Rafael Nadal who coasted into the third round in straight sets.

Svitolina, 21, will face two-time Wimbledon champion Petra Kvitova for a place in the semi-finals. Wimbledon champion Williams was clearly hampered by the shoulder injury which forced her to skip the Montreal event.
    
 At one point, she screamed in pain, "shoulder, shoulder".

She even served up five double faults in the seventh game of the second set as she suffered her earliest ever loss at an Olympic singles event.

"I didn't expect that I could win. I stayed in the moment, relaxed and went for my shots especially in second set because it was really tight," said Svitolina, the world number 20.

Muguruza's post-French Open slump continued when she slipped to a worrying 6-1 6-1 loss to Puig, the world number 34. Puig, 22, closed in on what would be only Puerto Rico's ninth Olympic medal and will face Germany's Laura Siegemund for a semi-final spot.

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