Williams, who won last week's tournament in Seoul, was pushed to a tiebreaker in the first set but still came away with a 7-6 (7-4), 6-3 win over Alicia Molik.
Carolina Wozniacki, who defeated Klara Zakopalova 6-3, 2-6, 7-6 (7-3), is next up in the draw for Williams.
The other semifinal Friday pits fifth-seeded Virginie Razzano versus No. 8-seeded Flavia Pennetta. Razzano, like Williams boasting an eight-match winning streak having won a tournament in China last week, made the semifinals with a 6-2, 6-2 romp over Camille Pin. Pennetta's quarterfinal win came at the expense of second-seeded Sania Mirza 6-4, 6-4.
The 2007 Japan Open women's champion will earn $25,840.