Williams, still rolling after last week's Masters series victory in Toronto, reached her first career Cincinnati final after holding off a charge from defending champion Li Na to record a 7-5, 7-5 triumph.
China's Li had reached the semis after her opponent in Friday's quarterfinal match, Agnieszka Radwanska, pulled out of the tournament to attend the funeral of her grandfather in Poland.
Belarusian Azarenka, meanwhile, had to work much harder in her semifinal, rallying from a set down to overcome Serbia's Jelena Jankovic 4-6, 6-2, 6-3.
Azarenka has a career 2-12 record against the American superstar, but has split a pair of finals against her this season, downing Williams in Qatar before losing to her in the Rome Masters.
Williams also bested Azarenka in the finals of last year's U.S. Open.