Garcia recovered remarkably well from a tough clash with Daria Kasatkina from the night before - a 2hr 27min affair that ended in a third-set tiebreak - and looked sharp against the fifth-seeded Sakkari.
The 29-year-old now has a chance to become the first French champion at the WTA Finals since Amelie Mauresmo in 2005.
Garcia is 2-2 head-to-head against Sabalenka but won their sole previous meeting this season, a three-set semi-final showdown en route to lifting the trophy in Cincinnati three months ago.
"Aryna is a very powerful player, huge serve, huge forehand, and she looks like she's playing some great tennis here so far," said Garcia, who is chasing her fourth title of the season.
"We know that she's very, very always showing a lot of emotion on court, going for her shot. I played her a couple months ago in Cincinnati. And, I mean, it's different. I would like to put pressure on her obviously, and to be able to use their ball as well. But I will try to play my game."