The ninth-seeded Stosur broke Williams' serve five times en route to a dominating win, her initial Grand Slam title.
She became the first Australian woman to win a Grand Slam title since Evonne Goolagong Cawley won Wimbledon in 1980.
Williams, meanwhile, looked tired less than 24 hours after ousting top-ranked Caroline Wozniacki in the semifinals late Saturday night.
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