Wimbledon champion Muguruza proved much stronger on serve than the defending champion, advancing to Sunday's final in less than 80 minutes.
The Spaniard ended a run of six consecutive defeats against Czech Pliskova.
Muguruza will play either Romanian Simona Halep or American Sloane Stephens in the final.
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In the day's second match, Simona Halep qualified for the final with a victory over home favourite Sloane Stephens.
The Romanian, ranked number 2 in the world, won 6-2 6-1.
Aug 19 - Results from the Cincinnati Open Women's Singles Semifinal matches on Saturday
2-Simona Halep (Romania) beat Sloane Stephens (U.S.) 6-2 6-1
4-Garbine Muguruza Blanco (Spain) beat 1-Karolina Pliskova (Czech Republic) 6-3 6-2
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