Plíšková was playing in her first event since moving to the top of the world rankings last month.
Wozniacki, the 2010 champion, needed 80 minutes to eliminate Stephens and reach her sixth final of 2017. She beat Petra Kvitova, Angelique Kerber and Lucie Safarova to reach the semi-finals. But her efforts were undone by too many unforced errors and Wozniacki calmly marched through the first set.
The Danish sixth seed jumped out to a 3-0 lead in the first set then broke Stephens again to take a commanding 5-1 lead.More news: Here's how you can see the Perseid Meteor Shower Saturday night
Wozniacki had to battle several rain delays, with four in total causing problems on Friday, but she was able to maintain her composure and secure a 7-5 6-7 (3) 6-4 victory.
Wozniacki will play defending champion Simona Halep or Elina Svitolina for the championship on Sunday at Toronto's Aviva Centre.
The pair will play twice in a day after rain disrupted the schedule.More news: UP: CM Yogi asks madarsas to record I-Day celebrations on camera
The second-seeded Federer is seeking a third Rogers Cup title to go with wins in 2004 and 2006.
After beating Spain's Rafael Nadal yesterday, the 18-year-old wild card entry overcame Frenchman Adrian Mannarino 2-6, 6-3, 6-4. "I just want to give my best, enjoy the match, and try to fight".More news: The Precision Drilling Corporation (PDS) Earns "Hold" Rating from Piper Jaffray Companies
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