The Grio, Osaka, who also beat Williams in the chaotic 2018 US Open final, reached her fourth major title match and stretched her winning streak to 20 matches by claiming the last eight points.
Osaka will face 22nd-seeded American Jennifer Brady for the championship on Saturday. Brady beat No. 25 Karolina Muchova 6-4, 3-6, 6-4 in the other semifinal.