Spending Christmas at home inspired Fenninger to clock a combined time of two minutes 13.09 seconds.
The 23-year-old super-combined world champion, who won her maiden World Cup race in Leinz exactly a year ago, defeated World Cup leader Tina Maze by 1.10 seconds with France's Tessa Worely a further 0.08 of a second behind in third place.
"It was like dancing on the snow," said Fenninger of her immaculate performance in the closest ski resort to the Austrian capital Vienna.
"This is a real leap forward for me because I never managed anything good here in the past. It was hard because it was the first time I was in the lead after the morning run.
"But after my fourth place in Courchevel and my second place in Are, confidence is back. My results are clearly on the way up.
It was Maze's 10th top three placing of the season and the Slovenian again increased the gap in the overall World Cup standings. She now leads second-placed Maria Hoefl-Riesch of Germany by 417 points.
Fenninger was the only skier to fully master the flat course on which it was difficult to gather speed, especially in the windy conditions prevailing on Friday.
"Today, Anna wasn't skiing, she was flying. She was just impossible to beat," said Maze, who won the first four giant slaloms of the season.
A slalom is scheduled on the same course on Saturday and Maze will again be among the favourites in the absence of injured world champion Marlies Schild.