Former world motor-racing champion Michael Schumacher has undergone emergency surgery for a head injury suffered while skiing in the French Alps.
The 44-year-old German was wearing a helmet when he fell and hit his head on a rock while skiing with his 14-year-old son on Sunday morning.
The seven-times Formula 1 champion remains in a critical condition in a Grenoble hospital on Monday after suffering a serious cranial trauma.
Though he was able to walk from the scene complaining only of being a little shaken, within an hour his condition had gravely worsened, the BBC reports. Experts say that his brain may well have begun to swell, requiring an urgent operation to relieve the pressure.
Schumacher is likely to have been kept in an induced coma as doctors monitor his vital signs.
The retired motor-racing driver had been skiing off-piste in the upmarket Meribel resort when the accident happened.
He was flown to a local hospital, then, an hour later, to the better-equipped facility in Grenoble in south-east France. Gerard Saillant, a top surgeon and brain specialist from Paris, was rushed in to oversee the treatment, AFP reports.
The resort's director, Christophe Gernigon-Lecomte, had said just after the accident that Schumacher was "conscious but a little agitated", suggesting that he had not received life-threatening injuries. However, when he then fell into coma, doctors realised the damage was worse than initially feared.
Two mountain police officers who gave first aid to Schumacher said he was suffering "severe cranial trauma" when they got to him and a helicopter was brought in to evacuate him within 10 minutes.
Friend Jamie Fox said he believed Schumacher had been skiing in between two runs where he had also been a few days earlier. Mr Fox said there had only been a light coating of snow recently and Schumacher may not have been able to see the rocks.
Schumacher lives with his family in Switzerland and celebrates his 45th birthday on 3 January. His wife Corinna was at his side with his two children at the Grenoble hospital.
Formula 1 drivers from around the globe have expressed their shock at the news. Police are investigating the circumstances of the accident.
Michael Schumacher, who won the last of his world titles in 2004, definitively retired in 2012 in the Brazilian Grand Prix, in which he finished seventh, after an abandoned attempt to quit six years earlier.
Since his debut in 1991, the German towered over the sport, winning more Formula 1 world titles and races than any other. He had a record 91 wins and is one of only two men to reach 300 grands prix, AFP reports.
His duels in his heyday with Damon Hill and Jacques Villeneuve, fired by an unquenchable competitive spirit, have gone down in Formula 1 folklore.
Schumacher was born in 1969 near Cologne, Germany, the son of a bricklayer who also ran the local go-kart track, where his mother worked in the canteen.