Plíšková was playing in her first event since moving to the top of the world rankings last month.
Sixth seed Wozniacki, the champion in 2010, saw off unseeded American Stephens 6-2 6-3 in the first semi-final in Toronto.
Sixth-seeded Caroline Wozniacki defeated Sloane Stephens 6-2, 6-3 on Saturday afternoon to reach the final of the Rogers Cup women's tennis tournament. She beat Petra Kvitova, Angelique Kerber and Lucie Safarova to reach the semi-finals. But Wozniacki struck again a few games later, and served out the match after a brief rain delay at 5-3.
Speaking after the victory, the 27-year-old said: "Almost every tournament I've played this year there has been a rain delay so at this point I just have to laugh because I feel like it is following me".
"Yesterday was really tough, waiting like six or seven hours here on site so I was a little bit exhausted", Halep said.
epa06141136 Robin Haase of the Netherlands in action against Roger Federer of Switzerland during the ATP Rogers cup men's semi final in Montreal, Canada, 12 August 2017. Halep defeated Garcia 6-4, 6-2. She'll face either Elina Svitolina or Simona Halep for the title. "I just want to give my best, enjoy the match, and try to fight".