It was an impressive display by the reigning US Open champion, who booked her place in Saturday's showpiece with a 3-6, 6-2, 6-1 triumph, just 18 days after beating the Belarusian in straight sets in the second round at Indian Wells.
"I am really pleased with the way I fought back after the first set," said Stephens, 25, who is chasing the sixth title of her career.
"You never know what can happen, I have really had to scrap it out, so to reach the final makes it so special.
"For the first time I listened to my coach and I was able to get back into it. I just had to stay focused on myself."
Azarenka 28, has enjoyed her strongest run since returning to the tour full-time having taken a leave of absence to deal with a child custody issue and with a current ranking of 186 was the lowest ranked of seven unseeded players to reach a Miami semi-final.
She will return to the top 100 following this run though will be bitterly disappointed not to have got the opportunity to win a fourth title here after making such a solid start.
Stephens has endured a slump since triumphing at Flushing Meadows last September but has finally found her form at just the right time.
Despite falling behind, the American, who will enter the top 10 for the first time next week when the WTA rankings are announced, upped her game while Azarenka, hamstrung by six double faults, lost her way.
The two-time Australian Open champion appeared to be injured as she sloped off court at the end.
Thursday's other semi-final sees American qualifier Danielle Collins take on French Open champion Jelena Ostapenko.