The seventh-seeded Doi never faced a break point in her 62 63 defeat of Hsu, the third time in three encounters she has come out on top in straight sets.
"Today I played really well, especially my service game," Doi said. "I was able to hit the ball right where I was aiming."
Doi, whose semifinal showing in Osaka last month was her best career outing, is into another final four in Taipei by virtue of quarterfinal opponent and fellow Japanese Ayumi Morita's withdrawal because of a right knee injury.
Setting up a quarterfinal clash were second-seeded Olga Govortsova and fifth-seeded Chang Kai-Chen. Govortsova beat Irina Khromacheva, 64 62. Chang, who is coming off her first WTA final in Osaka, endured the day's biggest test but was a 76(5) 63 winner over Caroline Garcia.
Several Taiwanese players thrilled local fans by making the doubles semis. Kao Shao-Yuan and Lee Hua-Chen, Chan Hao-Ching and France's Kristina Mladenovic and Chan Chin-Wei and Japan's Kimiko Date-Krumm all won.
In a clash featuring players 23 years apart, Mladenovic will take on Date-Krumm in singles quarterfinal action on Friday.