The Wimbledon champion, who moved to the top of the WTA rankings earlier this month, emerged a 6-2 6-4 victor over the ninth-seeded Garcia to edge closer to the ninth final of her career.
Sky Bet make Muguruza 11/10 to win the title and the Spaniard will next face defending champion and 4/1 chance Caroline Wozniacki.
The Dane reached the semi-finals in Tokyo for a fifth successive year but not before surviving a big scare against Dominika Cibulkova.
Wozniacki, the third seed, had to save two match points before finally going through 3-6 7-6 (7/5) 3-1 when the fifth-seeded Slovakian was forced to retire with a thigh injury.
Angelique Kerber is Sky Bet's 11/4 second favourite after she beat another former world number one in Karolina Pliskova 7-6 (7/5) 7-5.
Czech Pliskova surrendered top spot to Muguruza having herself claimed it off Kerber in July - and a repeat of the 2016 US Open final, which the German also won.
Standing in Kerber's way next is 6/1 tournament outsider Anastasia Pavlyuchenkova, who beat Barbora Strycova 5-7 6-3 6-1.
In China, Yanina Wickmayer's impressive run at the Guangzhou Open came to an end as she was beaten by Aleksandra Krunic in the semi-finals.
The Belgian had beaten top seed Peng Shuai and number six Alize Cornet in reaching the last four, but she was dumped out 6-3 6-3 by Krunic.
The 24-year-old Serb is through to her maiden WTA final and will take on second seed Zhang Shuai for the title, with the home hope - the 2013 champion - edging past Evgeniya Rodina 7-6 (9/7) 6-3.
In the Korea Open in Seoul, fourth seed Sorana Cirstea suffered a surprise straight-sets defeat to Thai qualifier Luksika Kumkhum in the quarter-finals, the Romanian going down 6-3 6-1.
That left top seed and Sky Bet's 10/11 favourite Jelena Ostapenko as the only seeded player in the semi-finals, with the French Open champion proving far too good for Veronica Cepede Royg in a 6-3 6-1 victory to set up a showdown with Kumkhum.
Also through to the last four are Richel Hogenkamp and Beatriz Haddad Maia, who defeated Priscilla Hon and Sara Sorribes Tormo respectively.
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