GRENOBLE: Doctors treating Michael Schumacher said that the Formula One legend has undergone a second operation following his life-threatening ski accident but warned he is “not out of danger”.

Surgeons said there had been a “slight improvement” in his condition and that they had “gained some time” by performing a successful second operation on the seven times world champion on Monday night.

His family is at the hospital in the French Alpine city of Grenoble where the former racing driver remains in a coma after he fell and slammed his head on a rock while skiing off-piste on Sunday.

News of the accident stunned the world and racing stars joined German Chancellor Angela Merkel and legions of fans in expressing their hopes for his recovery.

The second operation Monday was to remove a blood clot which was putting pressure on the brain, doctors said.

Surgeons only went ahead with the operation after consulting Schumacher’s family, who took the “difficult decision” to agree to a new procedure.

However, Jean-Francois Payen, head of the intensive care unit, told reporters at a press conference Tuesday that Schumacher was still in danger.

“We cannot speculate on the future,” he said.

Doctors claimed they were “surprised” by the improvement in Schumacher’s condition but he was still “critical” and remained “fragile”.

Doctors have said that Schumacher, who is due to turn 45 on January 3, has age and physical fitness on his side but stressed it was too early to say if he would pull through.

He has been put in a medically induced coma to spur recovery. The coma reduces the patient’s temperature to around 35 degrees Celsius (95 degrees Fahrenheit) to reduce swelling.

By being unconscious, the brain is also switched off to sounds, light and other triggers that cause the organ to use up oxygen as it processes the stimuli.

A source close to the investigation into the off-piste accident at the posh ski resort of Meribel told AFP that Schumacher's helmet, which medics say saved his life, was smashed “in two” by the impact.

The German newspaper Bild also quoted a rescuer as saying the split helmet was “full of blood”.

Schumacher's family in a statement expressed their thanks to the doctors who they said were doing “everything possible to help Michael” and to well-wishers around the world. Damon Hill, who fought several memorable on-track battles with Schumacher, said he was “praying” for his former rival.

Merkel was “extremely shocked” by the incident, her spokesman Steffen Seibert told reporters.

Formula One quadruple world champion Sebastian Vettel, 26, who has said Schumacher was his childhood idol, said: “I am shocked and I hope that he'll be feeling better as soon as possible.

Schumacher, who won the last of his world titles in 2004, towered over the sport since his debut in 1991, 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.

His duels in his heyday with 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 won his first Formula One grand prix.

He joined Ferrari in 1996 and went from strength to strength over the next decade, dominating the podium, before retiring aged 37.

But the father of two could not resist the lure of the track and in 2010 he came out of retirement, signing a deal with Mercedes before quitting for good in 2012.