Germany's Kerber, seeded fifth, bested the Czech Republic's Petra Kvitova, who was looking to score back-to-back wins after last week's victory in the Rogers Cup.
Kerber needed three sets but ousted Kvitova 6-1, 2-6, 6-4, securing her place in the Cincinnati final.
There she'll meet ninth-seeded Li of China, who prevailed over wild card Venus Williams 7-5, 3-6, 6-1 on the hard courts at the Lindner Family Tennis Center.
Li made her second WTA final in two weeks -- she lost to Kvitova in Monday's Rogers Cup finale. She's seeking her first crown of the season, while Kerber is 2-1 in three WTA finals so far this year.