US ski star Lindsey Vonn suffered a heavy crash during the opening race of the world championships here on Tuesday, but it was still unclear whether she suffered any serious injuries.

The 28-year-old two-time world champion and Olympic gold-medallist landed on the side of her ski after a jump during the opening super-G race here, and flipped over before crashing into a safety gate.

Emergency services were quickly on site, and Vonn was evacuated by helicopter to a nearby hospital.

Austrian television reported possible leg injuries.

The four-time World Cup winner was looking to recapture her super-G world title from 2009 and had been leading before her crash.

"I thrive off of challenges and this is just a new challenge so I'm excited about it and definitely getting ready to get my game face on," she said ahead of the races.

Tuesday's race began with a three-and-a-half-hour delay due to fog, which seriously limited visibility and made racing down the course, where athletes can reach a speed of 120kph highly damgerous.

Some fog continued to hinder the racers even after the start however, including Germany's Viktoria Rebensburg.

Tina Maze was in the lead with a time 1min 35.39sec when Vonn had her fall.