With a 6-3, 7-5 victory, Andreescu, a 19-year-old Canadian, captures her first Grand Slam title and denies Williams her 24th.
The American overpowers China's Wanq Qiang, firing off 25 winners compared with zero from her opponent.
Williams beat Petra Martic on Sunday after No. 2 Ashleigh Barty and No. 3 Karolina Pliskova lost in the fourth round.
When it comes to her health this year, the No. 1 seed just can't catch a break.
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