The Swede, lying 90 points behind Shiffrin in the slalom standings, was first out of the gate and clocked 46.27 seconds in warm, overcast conditions.
Shiffrin, seeking a third consecutive crystal globe in the discipline, was second at 0.07sec and will be confident of taking home the title on the Roc de Fer piste on rapidly deteriorating snow.
The race for the overall World Cup title also heated up, with defending champion Anna Fenninger of Austria deciding to race the slalom in a bid to put some pressure on Slovenian all-rounder Tina Maze.
Fenninger, who last participated in a slalom in December 2011 and has never finished better than 19th, finished 22nd at 2.53sec.
Maze, however, sat in fourth at 0.61sec and on course to score some serious points to potentially overtake Fenninger, currently 32pts in the lead, atop the overall standings.
The women round off the season with the giant slalom on Sunday.