Al Unser, a four-time winner of the Indianapolis 500, broke his collarbone and right thigh and suffered multiple rib injuries Thursday in a crash during practice for Sunday's Marlboro 500 at Michigan International Speedway.
Observers said Unser, 51, from Albuquerque, N.M., hit the wall after his car's front suspension broke as he drove through Turns 3 and 4 on the two-mile, high-banked oval.
Unser, only a part-time performer on the Championship Auto Racing Teams circuit this season, was taken to W.A. Foote Hospital in nearby Jackson, Mich. Track officials said he was conscious and alert.
Jim McGee, team manager for the Patrick Racing team, for whom Unser was driving, said: "I was talking to Al on the radio as he was in the car. He was stuck in the car and (CART's safety crew) had to cut him out.
"Al said he felt the car bottom in the corner and then it went straight into the wall. Al had tested about 170 miles this week at Indy and everything seemed very good with the car."
Unser's new Alfa Romeo-powered Lola was badly damaged and the safety crew tore it apart further while getting Unser out of the wreckage.
Unser's top lap of the opening practice session was 208.412 m.p.h.
Al Unser Jr., was on the track at the time of the crash.
"I didn't know who it was when I first saw that something definitely dug a line in the asphalt all the way up to the wall," said the younger Unser, the CART-PPG point leader. "When I saw it was Dad, I slowed down quite a lot and I saw his head moving.
"The (right front) tire was right up against his hip area, but there was no hole made by the tire in the tub. It didn't look too serious when I went by."
He spoke with his father in the ambulance before it left the track.
"I asked him if he knew where he was, and he did," Unser Jr. said, "and I asked him what happened and he said, 'The car broke.' "
Unser was transferred to Methodist Hospital in Indianapolis where he underwent leg surgery. He was listed in fair condition and was expected to be hospitalized five to seven days, said Trudy Doyle, administrator of patient care services at Methodist.