Ana Ivanovic
The 2004 junior Wimbledon runner-up, ranked 155 and playing only her fourth match in the main draw of a WTA event, matched the former world No.1 from first ball to last.
Still 16, Ivanovic used the confidence gained from winning all 31 of the Challenger matches she has played this year to stretch Williams to the limit.
Although Williams broke to lead the first set 3-1, Ivanovic broke back in the seventh game, and then escaped a set point at 6-5 when Williams netted a weak return.
In the tiebreak, Ivanovic earned five set points, before a backhand error gave Williams the set on her third set point.
In the second set, Williams was 4-2 up before being caught again, then Ivanovic led 6-3 in the tiebreaker. On her third set point, however, she netted the ball and moments later Williams was the winner.