Williams reeled off 11 aces en route to a 6-4, 6-3 decision over Sharapova for her third career win at the season-ending event, which she won in 2001and 2009.
The American star recorded her 12th straight win over a first- or second-ranked opponent and ran her winning streak against Sharapova to nine matches.
Sharapova dropped to 3-6 in finals this year.
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