Champ Car driver Cristiano da Matta was flown to hospital for emergency surgery after his car collided with a deer in an open test session at Road America in Elkhart Lake, Wisconsin on Thursday.
The 32-year-old Brazilian, who was airlifted to Theda Clark Memorial Hospital in Neenah, Wisconsin, suffered a head injury and was in intensive care, his team RuSPORT said in a statement.
"A head CT scan showed Cristiano had suffered a subdural hematoma," Champ Car director of medical affairs Dr Chris Pinderski said.
"He was urgently taken to the operating room for surgical removal of the hematoma and is now being observed in the intensive care unit."
Champ Car (CART) winner in 2002, da Matta lies sixth in this season's series standings with 134 points.
The Brazilian returned to Champ Car last year after competing for the Toyota Formula One team in 2003 and 2004. He started 28 Formula One races for Toyota with sixth place his highest finish.