Top-ranked Novak Djokovic was knocked out of the Olympic men's singles tennis tournament in the first round Sunday by Juan Martin del Potro.

Del Potro, former US Open champion, is ranked 145 in the world after an injury-hit few years, stunned the Serb with a powerful display to win 7-6 (7-4), 7-6 (7-2) in the biggest upset in the tennis tournament.

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