Jules Bianchi's father says he is "less optimistic" that his son will recover from head injuries sustained in a crash at last year's Japanese Grand Prix.
"In general, progress needs to be made in the first six months," Bianchi said.
"It has been nine months now and Jules has still not woken up."
Frenchman Jules Bianchi, 25, continues to receive treatment for his severe injuries in hospital in his home city of Nice.
Philippe Bianchi added: "As time goes by, I am less optimistic than I might have been two or three months after the accident when we could have hoped for better progress."
Bianchi admitted that if Jules did awake from his coma, it could lead to added difficulties.
He revealed that, prior to his son's accident, the two had spoken about the condition of seven-time Formula 1 world champion Michael Schumacher, who suffered severe head injuries in a skiing accident in December 2013 and faces a long road to recovery.
Philippe Bianchi said: "If he had a severe handicap, we are convinced that is not what Jules would want.
"We talked about it. He discussed with us that if one day he had an accident like that of Michael Schumacher, that even if his only handicap was not being able to drive, he would have a lot of difficulty living. Because it was his life."
Jules Bianchi joined Marussia in 2013 and raced in 34 grands prix, scoring two championship points in last year's Monaco Grand Prix.
Marussia came close to collapse over the winter but were rescued and are racing as Manor in 2015.