Canadian driver James Hinchcliffe was taken to hospital for surgery after his car smashed into a wall and briefly set on fire in the fourth dramatic crash in under a week at the Indianapolis 500.
IndyCar said the Schmidt Peterson Motorsports driver was "alert" after the crash but had a thigh injury.
The incident was reported to be the result of a mechanical failure, with Hinchcliffe going straight into a wall.
The 28-year-old is in a stable condition in intensive care.
Rule changes have been introduced amid concerns over the cars' designs.
In an effort to reduce speed and increase downforce, the cars - which can travel at up to 230mph - must now run race-level aerodynamics and engine-boost settings during qualifying.
Hinchcliffe had just completed a lap at 221.3 mph when the accident occurred.
One week, four crashes
Three-time Indy 500 winner Helio Castroneves lost control of his car before it hit the perimeter wall and went airborne during Wednesday's practice.
The following day, fellow Chevrolet driver Josef Newgarden lost control going into the first turn at Indianapolis Motor Speedway before the car flipped and rested on its side after hitting the wall.
On Sunday, their CFH Racing team-mate Ed Carpenter was unhurt after his car crashed into a wall and flipped over during practice.
Hinchcliffe's accident followed a day later, as the drivers prepared for the 500-mile race on 24 May.
British driver Pippa Mann, of Dale Coyne Racing, also crashed this week but said her car's design had not been a factor, describing the incident as "just one of those things".