World No.62 Popyrin extended a brilliant run of upset victories as he earned his spot in the decider with two victories in one day.
The 25-year-old Olympian from Sydney defeated last week's Washington title winner Sebastian Korda 7-6 (7-0) 6-3 in his semi-final on Sunday night.
Hours earlier he won a tight quarter-final battle to oust fourth-seeded world No.6 Hubert Hurkacz 3-6 7-6 (7-5) 7-5 after coming within two points of defeat.
Alexei Popyrin reacts during his win over world No.6 Hubert Hurkacz. (AP PHOTO)
Popyrin had already overcome 11th seed Ben Shelton and seventh seed Grigor Dimitrov in previous rounds.
"It's an amazing achievement for myself, to be honest, and sometimes you have to pat yourself on the back," Popyrin said about reaching the final.
"I'll do that tonight, but tomorrow it's all hands on deck and back to work."
Popyrin will be seeking the third, and biggest, title of his career against fifth-seeded Russian Rublev on Monday night (Tuesday AEST).
He's already set to rise to a career-high world No.30 on the back of his feats this week and a win would propel him to No.23 in the world as well as locking up an all-important seeding for the US Open starting in two weeks.
No Australian has won a Masters 1000 series title since Lleyton Hewitt successfully defended his Indian Wells crown in 2003.
Popyrin is following in the footsteps of countryman Alex de Minaur, who made the final of the Toronto Masters 1000 event last year, before losing to Jannik Sinner.
The Montreal title won't come easily.
World No.8 Rublev has victories this year in Madrid and Hong Kong among 16 career titles and he made relatively quick work of Italy's Matteo Arnaldi, winning 6-4 6-1 in a rain-interrupted first semi-final earlier in the evening.
The 26-year-old had beaten top-seeded Australian Open champion Sinner 6-3 1-6 6-2 in their quarter-final on Saturday night and is 21-7 in hardcourt matches this season.
Popyrin beat Rublev in the first round at Monte Carlo at the start of the clay season and arrived in Montreal after winning two Olympic matches on clay at Roland Garros.
But he has adapted quickly to hardcourt and has spent nine hours and 54 minutes on court on his way to the final.
Rain previously washed out play Thursday night and all of Friday as remnants of Tropical Storm Debby rolled through, putting the event behind schedule.
With The AP
