Sinner outlasted Paul in a slugfest that was a showcase of power from both sides and at one point saw the duo engage in a 46-shot rally that marked the longest of the week in Toronto.
"It was a fight obviously," said Sinner, who saved eight of 11 break points in the win. "He was playing great as I do and at the end, you know match point, when I got there I was praying please. But thank God I won in two (sets)."
Sinner and Paul twice exchanged breaks in the opening set but it was the Italian who held serve when it mattered most as he took a 5-4 lead before clinching the set with another break.
Sinner led 2-1 in the second set when Paul called for the tour physio who worked on his right hip while he lay sprawled on his left side in discomfort.
Paul was unable to hold serve in the next game, handing over the break with a double fault, before a determined Sinner went on to consolidate for a commanding 4-1 lead.
From there, the Italian continued to display great length and pace on his ground strokes and turned aside three break points, including one during a 46-shot rally, for a 5-2 lead that sent him on his way.
"I tried so hard to win that point," Sinner said. "Just tried to play one more ball in the court than him and that was I guess the solution."
De Minaur overcame blustery conditions to secure a breakthrough victory over Davidovich Fokina and reach the first Masters 1000 final of his career.
De Minaur, who was fresh off an upset over second seed Daniil Medvedev, delivered a simple gameplan in rather tricky conditions, while Davidovich Fokina struggled to find any sort of rhythm throughout the 78-minute match.
"First thing I knew once I stepped out on court it wasn't going to be pretty, so I didn't expect perfect tennis from my side of the court today," said De Minaur, who wrote "so windy" on an on-court camera lens after the match.
"I just told myself to stay positive, keep competing no matter what and you never know what's going to happen.
"So I thought I had a great mindset today. Didn't play the best of tennis, but I did what needed to be done today in these conditions."
De Minaur wasted no time setting the tone against Davidovich Fokina as he broke the Spaniard twice en route to a 5-1 lead before serving out the opening set on his first opportunity.
The Australian maintained the pressure as he broke Davidovich Fokina four times to build a 5-2 lead in the second before sealing the match with his seventh break.
"I've had a hell of a week. It's been the breakthrough that I always knew I had inside of me so it's great to show it and play for a final," De Minaur said.