Brazilian Cristiano da Matta had surgery to remove a ruptured blood vessel in his head after a collision with a deer during Champ Car testing.
The former Toyota F1 driver, 32, is in intensive care after the crash at the Road America track in Wisconsin.
Da Matta has 12 Champ Car victories and claimed the series title in 2002.
Eye-witness reports suggest he collided with the deer head on, with the animal hitting his head as his car crashed into the barriers at full speed.
The deer had run from the woods onto the Elkhart Lake track near turn six, track officials said.
Da Matta underwent emergency surgery on Thursday for a subdural hematoma at the Theda Clark Medical Center in Neenah, Wisconsin.
"Cristiano da Matta continues to recover from a serious head injury sustained yesterday during Champ Car Open Testing at Road America," Champ Car director of medical affairs Chris Pinderski said in a statement on Friday.
Da Matta competed in 28 grands prix for Toyota in 2003 and 2004, scoring 13 points.
The Toyota team issued a statement from the Hungaroring, venue for this weekend's Hungarian Grand Prix.
It read: "When he was with Toyota, Cristiano was always a fighter and everyone involved in the team wishes him the best as he fights for his recovery."
Da Matta's RuSport team have withdrawn his team-mate, Yorkshireman Justin Wilson, from the remainder of the testing session.