A two-time champion in Doha, Azarenka made adjustments in the second set and took a 5-1 lead in the third. Azarenka had 36 winners in a match watched by David Beckham.
"Venus was kicking my butt left and right, and I didn't know what to do. In the second set, I took control," said Azarenka, who won the event in 2012 and 2013 but withdrew last year because of a foot injury.
Williams entered the match with a 4-0 record against the Belarusian, who reached her first final in more than a year.
"From 5-1, she really went for her shots, she was really aggressive," Azarenka said of Williams. "So I just wanted to stay composed."
The 48th-ranked Azarenka will face No. 15 Lucie Safarova in the final on Saturday. She's undefeated in six matches against the Czech player, who beat Carla Suarez Navarro 6-3, 6-2 in the other semifinal.
Safarova needed only 77 minutes to stop Suarez Navarro, who committed 21 unforced errors.
"I worked really hard in the offseason and my level has been going up, so I'm enjoying playing," said Safarova, who has won all three matches against Suarez Navarro.
In the first set, the 2014 Wimbledon semifinalist broke Suarez Navarro's serve in the second game. Although Safarova dropped her serve in the seventh game, she broke back in the next game. In the second set, she excelled with her powerful ground strokes.
Safarova said her game plan was to "play aggressive and take her time away because she's very good in defense. And she can also, from her backhand side, play very strong. So I was trying to push more through the forehand."