​Mikaela Shiffrin was imperious in winning a third straight slalom title at the ski world championships on Saturday to retain her unbeaten record at major events.

The 21-year-old American showed no nerves extending her first-run lead to finish a huge 1.64 seconds faster than runner-up, and home crowd favourite, Wendy Holdener of Switzerland. Frida Hansdotter of Sweden was third, trailing by 1.75.

On crossing the finish line, Shiffrin gazed up for several seconds at the giant screen and then gulped in air on seeing her massive winning margin.

"I thought 'I think that was good but I don't know where the time is [on the screen,]"' Shiffrin told Swiss broadcaster SRF.

Swiss fans, who had loudly supported Holdener two minutes earlier, duly acclaimed the unbeaten champion.

"I knew Wendy had a good run because everyone was screaming so loud," said Shiffrin, who was filmed minutes before her decisive run sitting beneath an umbrella to find shade from the sunshine.

Erin Mielzynski of Collingwood, Ont., was the top Canadian in 15th place. Marie-Michele Gagnon of Lac-Etchemin, Que., was 22nd, Toronto native Ali Nullmeyer was 27th and Mikaela Tommy of Wakefield, Que., did not finish her second run.

Shiffrin's gold medal streak in slalom includes each world championships she entered, starting in 2013, and the 2014 Olympics.

Her victory gave the United States its first world title at St. Moritz in the 10th of 11 medal events.

On a clear day and in perfect racing conditions, Shiffrin was 0.85 faster than any rival second time down, after beating Holdener by 0.38 in the morning first run.

Holdener added slalom silver to her gold medal in the combined event from Monday. Both her medals were won after Swiss star Lara Gut crashed out of the championships with a serious knee injury.

Shiffrin also took silver in giant slalom on Thursday. The U.S. team's tally is three medals, with Lindsey Vonn also getting bronze in downhill.