The family of Formula One champion Michael Schumacher is at his bedside as he faces a "decisive" 48 hours in his fight for life.
The champion driver is in an induced coma following surgery after he suffered a severe head injury while skiing off-piste in the resort of Meribel in France on Sunday.
Doctors have refused to speculate on a prognosis for the 44-year-old German.
Professor Stephan Chabardes, the neurosurgeon who operated on him, said yesterday the "next 24 or 48 hours will be decisive", The Times reported.
"He was operated upon in a serious condition with a serious, severe brain trauma. We are following the evolution hour by hour," he said.
"He is in a critical condition. The operation was to evacuate the haematoma that could be evacuated. Now we need to protect his brain."
The seven-times champion was admitted to Grenoble Hospital in France suffering from intracranial haematoma - blood clots - bruising and swelling of the brain.
Since the operation he has been kept in a state of hypothermia by controlling his temperature to keep it at 34C-35C (93.2F-95F). He is under general anaesthetic.
Schumacher fell and hit his head on a rock while skiing off-piste, and was reportedly travelling at speeds of up to 60mph when the accident happened, The Times said.
Professor Jean Francois Payen, head of anaesthetics at the clinic where Schumacher is being treated, said the fact that he was wearing a helmet - which reportedly cracked because of the severity of the impact - probably saved his life.
He said: "For the moment we cannot say what Michael Schumacher's future is."
He added: "Someone without a helmet would not have got this far."
Schumacher had travelled with his family to Meribel in the French Alps for the festive season.
His 14-year-old son was reported to have been with him at the time of the accident..
Rescuers were on hand within minutes and he was initially conscious after the fall before deteriorating into a critical condition.
Professor Gerard Saillant, a French brain surgeon who operated on Schumacher inn 1995 following a crash and who flew to Grenoble yesterday, said his physical condition would help him, The Times reported.
"Someone of 70 is less likely to survive this sort of crash than someone who is 45, and someone like Michael who is in top condition is more likely to survive than someone else."
Schumacher has received an outpouring of support from the racing world, with former and current stars urging him to pull through.
German chancellor Angela Merkel said she was "extremely shocked along with millions of Germans" to learn of the accident.
Schumacher retired from F1 for the final time in 2012 after a three-season comeback with Mercedes.
Schumacher, who also raced for Jordan, Benetton and Ferrari, won the last of his world titles in 2004.
He won two with Benetton in 1994 and 1995 before moving to Ferrari and winning five in a row from 2000. The German has 91 career wins.