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Kvitova, who won tournaments earlier this year in St Petersburg and Doha, will face either Romanian Mihaela Buzarnescu or Italy's Camila Giorgi in the final as she looks for her first clay-court title since the 2015 Madrid Open.
The two-time Wimbledon champion has risen back into the world's top 10 this year after dropping down the rankings following a hand injury inflicted in a knife attack at her home in 2016.
The 28-year-old needed four set points to clinch the opener against Zhang in a tie-break, but raced through the second for the loss of just six points.