Second seed Raonic did not face a break point and used his dominating serve to beat unseeded American Donald Young 6-4 7-5 at the William H.G. Fitzgerald Tennis Center in an afternoon match on Saturday.
Pospisil, the 13th seed, had to work much harder to get past France's Richard Gasquet 6-7(5) 6-3 7-5 in an evening encounter that stretched for nearly two hours, 30 minutes.
It was a long day for Wimbledon doubles champion Pospisil, who had to return to the hardcourt early in the afternoon to play the third set of his quarter-final that was delayed by rain on Friday night.
He won that match 6-7(4) 6-3 6-4 against Colombian Santiago Giraldo, and also started his semi by losing a first set tiebreak to sixth seed Gasquet, before fighting back to prevail in a match that hung in the balance until almost the very end.
Pospisil trailed 0-3 in the third set before breaking his opponent twice.
Earlier, world number seven Raonic, in his first event since reaching the Wimbledon semi-finals, broke Young at the end of each set to advance to the 11th ATP final of his career, the first this year.
"I did the most important things," the 23-year-old told reporters. "I took care of my serve. I thought he played really well and I just put that pressure on him in the final games of the set. It paid off."
Raonic knows his country's tennis fans will be watching the final closely.
"We've set the bar for Canadian tennis what can be expected of future generations," he said.
On the women's side, Japan's Kurumi Nara will meet Russian Svetlana Kuznetsova in the final.
Nara recovered from a terrible start to beat New Zealand's Marina Erakovic 0-6 6-4 6-4, while sixth seed Kuznetsova bested compatriot Ekaterina Makarova 6-3 6-2.
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