Michael Schumacher, the retired seven-time Formula One champion who often braved death on the tracks, was fighting for his life Monday after an off-piste skiing accident in the French Alps. The German racing legend, who turns 45 at the end of the week, was helicoptered off a mountain in the upmarket Meribel resort Sunday after falling and slamming his head on a rock while skiing off-piste with his 14-year-old son. News of the accident stunned the Formula One community and racing stars joined German Chancellor Angela Merkel and thousands on Twitter in wishing him a speedy recovery. Initially described as "serious" but not life-threatening, his condition gradually deteriorated and the hospital where he was being treated eventually announced late Sunday that Schumacher was critical, had serious brain trauma and had undergone an emergency operation. In an update to reporters on Monday, doctors at the hospital in the southeastern city of Grenoble said that while it was still too early to make a prognosis on the famous patient, he was fighting for his life. "He is in critical condition, his condition can be described as life threatening," Jean-Francois Payen, head of the intensive care unit, told reporters. Stephan Chabardes, the professor who operated on Schumacher, said the former racer arrived in hospital on Sunday in an agitated state -- his arms and legs jerking uncontrollably -- and was not able to answer questions. His condition "rapidly deteriorated" and he fell into a coma, he told reporters. Payen said he was immediately operated on and still suffered from "serious and diffuse brain lesions", which indicates his injuries are not localised but more widespread. He added that Schumacher would not still be alive if he had not been wearing a helmet. "Given the violence of the impact, his helmet partially protected him. If someone had had this type of accident without a helmet, they would definitely not be here," he said. Schumacher's 'most difficult race' News of the accident made waves, shocking fans and leaders alike. Merkel was "extremely shocked" by the incident, her spokesman Steffen Seibert told reporters. "We hope... that his injuries will heal and he will recover." Racing stars and fans also rushed to express their sadness and sympathy. "I know you Michael, you're a man, you're the best... This ordeal is your most difficult race but I am sure that you will win it too," Italian racing driver Giancarlo Fisichella wrote on Twitter. "We're very upset. We know him really well. He's a fighter, we're crossing fingers that he will win this battle," said Michael Viehmann, president of a Schumacher fan club in the small German town of Kerpen, where the retired racer grew up. Schumacher's accident comes after several off-piste skiers died or were injured in the Alps, and on Sunday authorities in the Savoie department where Meribel is located asked skiers to be extra "vigilant". 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. He joined Ferrari in 1996 and went from strength to strength over the next decade, 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, though 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 deal with Mercedes, before quitting for good in 2012. His helmet had a message for fans: "Life is about passions -- Thank you for sharing mine."