Shiffrin leads in slalom
Italy's Azzola reacts during the 1st leg of the FIS Alpine World Cup Women's Slalom, on January 5, 2014 in Bormio
The 18-year-old, who garnered world gold and the World Cup trophy in slalom last year, clocked a time of 51.72sec to best her biggest rival, Austrian record skier Marlies Schild.
Schild had been leading along with Sweden's Frida Hansdotter before Shiffrin, who has two slalom wins this season, breezed down the floodlit slope.
Germany's Maria Hoefl-Riesch then made her own bid for the lead but couldn't compete with Shiffrin.
The two-time slalom World Cup trophy winner and current overall leader finished 0.90sec slower than Shiffrin.
Schild and Hansdotter were joint third at 1.09sec.
The night race was widely seen as a face-off between veteran Schild and Shiffrin, 14 years younger than the Austrian.
Both skiers have won two slaloms so far this season, with Schild entering the record books with a record 35th World Cup victory in the discipline in Lienz, the most ever won by a woman.
Flachau is the second last opportunity for the slalom skiers to measure themselves against the competition ahead of the Olympics in Sochi on February 7-23.
The second run is due to start at 8:45pm (1945GMT).