Grenoble (France), Dec 31 (IANS): Legendary Formula 1 driver Michael Schumacher has shown signs of improvement but is still critical at the Grenoble University Hospital, doctors said Tuesday.

The seven-time former World Champion underwent another surgical procedure but is not out of danger. The surgery, which took around two hours, was done to remove the largest of a series of blood clots in Schumacher's brain, according to media reports.

The 44-year-old is still under medically induced coma after he suffered a severe head injury while skiing off-piste in the resort of Meribel in the French Alps Sunday. He was reportedly travelling at 60 mph when he fell and struck his head against a rock. He is thought to have been saved by his helmet which split on impact.

Doctors have refused to give a prognosis for the former Ferrari driver, who turns 45 Jan 3. Schumacher retired from Formula 1 for the second time at the end of 2012.