That left Federer serving for the set and the 36-year-old made no mistake, holding to love in a game sealed by an ace.
"I'm happy - most happy that I'm actually really healthy going into the finals", said second seed Federer, who has won Masters titles in Indian Wells and Miami in 2017.
"I didn't even know if I could ever play tennis again", Haase said. They ended up beating me at a certain stages, like Roddick, Gonzalez and Soderling.
"Today, I felt that I didn't get into the rhythm of the match and I didn't feel the ball well", he said.
"I mean, I go from being not known to being so known in the tennis world, in Canada in general". I gave away a lot of breaks with double faults. He's a great fighter on the court.
After Wozniacki sent Pliskova packing, American Sloane Stephens saved three match points and muscled her way past Lucie Safarova in a three-set thriller.
Federer was more aggressive in the second set.
Federer's performance was also buoyed by strong serve-and-volley stylings. Federer controlled matters from that point.
The Swiss star comfortably overcame 12th seed Roberto Bautista Agut 6-4 6-4 in the quarter-finals, but he feels he will have to be on top form to see off occasional practice partner Haase in Montreal.
"Confidence can get you a long way, and if you're feeling fine physically and you are ready to fight mentally you have a lot of things in the bag", said the 19-times grand slam champion. However, from that moment, I wasn't the player I was before my injury.
Federer finally began to hit his spots on serve, while tightening up his game from the back of the court.
Argentina's Diego Schwartzman recovered after failing to take a game in the opening set to beat USA's Jared Donaldson 0-6 7-5 7-5.
The 36-year-old Swiss will next face unseeded Dutchman Robin Haase, a 4-6, 6-3, 6-3 victor over Diego Schwartzman of Argentina.
Federer won the Rogers Cup in 2004 and 2006 when it was held in Toronto.
In doubles semifinals, seventh-seeded Rohan Bopanna of India and Ivan Dodig of Croatia defeated sixth-seeded Raven Klaasen of the Netherlands and Rajeev Ram of the United States 4-6, 7-6 (8), 11-9; and fifth-seeded Pierre-Hugues Herbert and Nicolas Mahut of France downed eighth-seeded Oliver Marach of Austria and Mate Pavic of Croatia 7-5, 6-3.
Top-seeded Rafael Nadal played a late match Thursday.