Stephens, who also beat Azarenka in the second round at Indian Wells this month, took control of the match early in the second set and never looked back as she raced through the decider with ease.
"I just knew I had to keep fighting. In the first set I was down but I battled back so I knew if I did that in the second, I'd be right in there," the 25-year-old American said in an on-court interview.
"I ended up winning a lot of games in a row and I just tried to keep the momentum going."
Stephens will face either French Open champion Jelena Ostapenko, the sixth seed, or American qualifier Danielle Collins in Saturday's final at Crandon Park.
Azarenka's initial comeback after the birth of her child in December, 2016 was short-lived as she played two tournaments before a custody battle with her son's father prevented her from travelling with her child.
The Belarusian may have played little competitive tennis in recent years but she enjoyed a respectable run in Miami, including victory over Czech world number six Karolina Pliskova in the quarter-finals.
(Reporting by Frank Pingue in Toronto,; Additional reporting by Rory Carroll in Los Angeles, editing by Ed Osmond)
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