Williams improved to 50-3 this season and is 27-0 on clay. She will be seeking her 53rd career championship against Larsson, who is still looking for her first WTA trophy.
Williams dominated the first set Saturday but Zakopalova battled back and at one tied the second set 4-4.
"She stated playing better in the second set," Williams said. "She was more aggressive and I started making some errors on a few key points. I'm happy I could close it."