JEREZ, Spain (REUTERS) - Sebastien Vettel kept Michael Schumacher in his thoughts on Tuesday as Formula One returned to the track after a long winter break with the German great still in a coma one month after a skiing accident.

Red Bull's quadruple world champion, who struck up a close friendship with his compatriot before the 44-year-old retired in 2012, said he was still shocked by what had happened to the sport's most successful racer.

"I pray and hope he will just come back and a miracle will happen and he will be the person he has always been before," the 26-year-old told reporters on the first day of pre-season testing.

Schumacher, a seven times world champion and winner of a record 91 races, suffered brain injuries when he fell and hit his head on a rock while skiing off-piste in the French Alps resort of Meribel on Dec 29.

"I have known him for a long time and have been looking up to him all my life... he became a friend," said Vettel, who paired up with Schumacher in the annual Race of Champions event as Team Germany.

"He is still in a coma and we still don't know in what shape he will be when he wakes up, which is horrible for his family and close friends." Vettel said he had not visited the former Benetton, Ferrari and Mercedes driver in hospital in Grenoble so as not to add to unwanted media attention.

Schumacher's family have urged the media to respect his privacy and the hospital and his agent have not issued a medical update for some time.

Mercedes paid their own tribute to Schumacher when they unveiled their 2014 car at the Jerez circuit on Tuesday with the Twitter hashtag #KeepFightingMichael prominent on the side of the airbox.

"As a team we want to show we are thinking about him, and hoping for the best every day," German driver Nico Rosberg, Schumacher's former teammate, told reporters.