American thirteenth seed Stephens beat the previous world number one 3 6-2 6-1.
Belarusian Azarenka, now positioned 108th, took the initial two amusements of the second set however Stephens won 10 diversions in succession to turn the tie around.
In the interim, Pablo Carreno Busta beat Kevin Anderson 6-4 5 7-6 (8-6) to advance to the men's last.
Thursday's last-four match was the second time world number 12 Stephens has crushed trump card Azarenka this month, having won in straight sets in the second round at Indian Wells.
Three-time Miami Open champ Azarenka, 28, serenely won the primary set however hit four twofold blames in the second as the American overwhelmed.
Stephens will play the victor of Friday's semi-last between qualifier Danielle Collins - who vanquished kindred American Venus Williams in the last eight - and French Open champion Jelena Ostapenko of Latvia.
"I knew I needed to remain in it. Victoria is an awesome champion," said Stephens, 25. "I continued doing combating and I got a little energy going in the second set.
"Experiencing childhood in South Florida, this is so mind blowing to be in the last.
"I knew I needed to center around my side of the court and be exclusively centered around myself."