Michael Schumacher, the retired seven-time Formula One champion, was in a critical condition on Monday after suffering severe brain trauma in a skiing accident in the French Alps, doctors said.
News of Schumacher's accident stunned the Formula One community and his former teammates joined thousands on Twitter in wishing him a speedy recovery.
"My thoughts are with Michael Schumacher at this tough time. Michael more than anyone has the strength to pull through this," tweeted British F1 driver Jenson Button.
Schumacher's former teammate at Benetton Martin Brundle wrote on Twitter: "Come on Michael, give us one of those race stints at pure qualifying pace to win through, like you used to. You can do it." (Schumacher haunted by love of speed and risk)
He added that the German was "a crazy brave skydiving/bike racing daredevil".
Brazilian Formula One racing driver Felipe Massa said he was "praying for God to protect you my brother!! I wish you a speedy recovery Michael", the former Ferrari driver wrote.
The Twitter feed of Germany's Adrian Sutil, who drives for Sauber, read: "I hope Michael Schumacher will get well soon! All my best to him and his family." Romain Grosjean of Lotus tweeted: "All our thoughts to Schumi and his family! Hope you will recover soon #legend #Schumi."
Support also came from leading German sports personalities, ranging from the NBA to soccer. Dallas Mavericks power forward Dirk Nowitzki said his thoughts were with Schumacher, while Lukas Podolski, who plays for Arsenal in the English Premier League, tweeted: "Bad news from Switzerland: please get well soon, Michael Schumacher. All the best for you, my friend! #getwellsoon #MichaelSchumacher."
The German racing legend, who turns 45 on Friday, had been skiing off-piste with his 14-year-old son in the upmarket Meribel resort when he fell and hit his head on a rock.
He was "suffering a serious brain trauma with coma on his arrival, which required an immediate neurosurgical operation", according to the hospital in the southeastern French city of Grenoble where he is being treated.
"He remains in a critical condition."
Shortly after the accident Meribel resort director Christophe Gernigon-Lecomte said Schumacher, who had been wearing a helmet, was "conscious but a little agitated".
But when Schumacher 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.
A renowned Parisian neurologist, doctor Gerard Saillant, arrived at the Grenoble hospital in a police car to help take charge of the famous patient.
Schumacher's wife Corinna was at his side with his two children, the hospital said.
The hospital statement was signed by the facility's neurosurgeon, the professor in charge of its anaesthesia/revival unit, and the hospital's deputy director.
In an email to The Associated Press, Schumacher's manager Sabine Kehm said the champion German driver was on a private skiing trip and "fell on his head."
"We ask for understanding that we cannot give running updates on his condition. He wore a helmet and was not alone," Kehm said.
Police kept guard at the hospital's entrances as journalists and fans, some wearing the colours of the Formula One legend's former stable Ferrari, gathered outside awaiting news of his health.
Police have opened an investigation into the circumstances of the accident, the ski resort said.
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 One 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.
Schumacher's duels in his heyday with Damon Hill and Jacques Villeneuve, fired by an unquenchable competitive spirit, have gone down in Formula One lore.
Schumacher was born in January 1969 near Cologne, Germany, the son of a bricklayer who also ran the local go-kart track, where his mother worked in the canteen.
By 1987, Schumacher was the German and European go-kart champion and was soon racing professionally. In 1991 he burst into Formula One by qualifying seventh in his debut race in Belgium and a year later he was racing for Benetton, where he won his first Formula One grand prix in 1992.
After joining Ferrari in 1996, Schumacher achieved infamy by trying to ram Villeneuve off the road at Jerez in the last race of 1997, and was disqualified from the championship as punishment.
Over the next decade, he went from strength to strength, dominating the podium, before trying to retire the first time aged 37.
During his retirement he survived a horror accident that knocked him out when racing a motorbike in Spain.
That time he was released from hospital after just five hours.
But the father of two could not resist the lure of the track and in 2010 he signed a three-year deal with Mercedes.
But slower reflexes and a less competitive car meant Schumacher could not reproduce his former glory and he quit for good in 2012. His helmet had a message for fans: "Life is about passions -- Thank you for sharing mine."