Dominika Cibulkova underlined her Pan Pacific Open title credentials with a straight-sets win over Carla Suarez Navarro on Tuesday.
The fifth seed was beaten in the final of last month's Connecticut Open by Daria Gavrilova, but proved too strong for the Spaniard as the world number nine triumphed 6-3 6-3 to set up a meeting with Katerina Siniakova.
Siniakova produced a remarkable fightback to defeat Cibulkova's Slovakian compatriot Jana Cepelova 1-6 6-3 6-4.
Gavrilova's participation in Tokyo was ended in a 2-6 6-0 7-6 (7-3) loss to Magda Linette.
Frenchwoman Caroline Garcia, seeded ninth, downed Aliaksandra Sasnovich 6-4 6-3, while Olympic champion Monica Puig overcame a first-set wobble as she beat Hsieh Su-wei 2-6 6-1 6-1 to book a showdown with top seed Garbine Muguruza.
There were contrasting fortunes for American duo Shelby Rogers, who saw off Risa Ozaki 6-2 7-6 (7-5), and Madison Brengle, who fell to a 3-6 6-2 6-4 loss against Daria Kasatkina.
Teenager Cici Bellis was another American casualty, losing 4-6 6-0 6-4 to Anastasia Pavlyuchenkova, while Barbora Strycova recorded a 7-6 (7-4) 7-6 (7-2) victory over Magdalena Rybarikova.