Scott Barbour - Getty Images

South Africa's Denis Istomin (top) upset six-time men's champion Novak Djokovic (bottom) in the second round of tennis's first Grand Slam, the Australian Open, early Thursday morning U.S. time.

The five-set match took nearly five hours to play.  This is Djokovic's earliest exit in a Slam event since last year's Wimbledon, where he lost to American Sam Querrey in the third round.

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