Seven-time world champion Michael Schumacher turned 50 on Thursday with Formula One and his fans paying tribute to the sport's most successful driver with a mixture of celebration and sorrow.

The German great has not been seen in public since he suffered severe brain injuries in a recreational skiing accident in the French Alps five years ago.

"Our champ turns 50 today. We're all with you Michael #KeepFighting," said Ferrari, celebrating a driver who won five titles in a row for the Italian team between 2000 and 2004.

"Michael, you've always been a fighter and you always will be," said former Ferrari boss Jean Todt, the president of the sport's governing body the International Automobile Federation (FIA), on Twitter.

Schumacher now lives a sheltered existence at the family's mansion by the shores of Lake Geneva in the town of Gland, Switzerland, with his wife Corinna and children highly protective of his privacy.

"You can be sure that he is in the very best of hands and that we are doing everything humanly possible to help him," the family said in a rare statement on Wednesday that also announced the launch of a Michael Schumacher app.

TREMENDOUS IMPACT

Thursday's focus was on celebrating a champion who holds the records for most titles and wins (91), even if Britain's five-time champion Lewis Hamilton is closing in.

The Formula One website declared 'Schumacher Week', with stories and interviews looking back on the career of a man who retired in 2012 after a three-year comeback with Mercedes.

"Michael has had a tremendous impact on Formula One," said Mercedes team boss Toto Wolff in a statement.

"Not only did he set an incredible record - a record that is yet to be beaten - but he also shaped and changed the sport forever," added the Austrian of a driver whose Formula One debut with Jordan at Spa in 1991 was funded by Mercedes.

"As a driver, Michael took Formula One to a whole new level with his attention to detail and his technical knowledge.

"He did everything with great determination, from his engineering debriefs to his physical training, and was always searching for new ways to improve his on-track performance."

Formula One's managing director for motorsport Ross Brawn, a key player in Schumacher's career at Benetton and Ferrari and also as Mercedes boss before Wolff, spoke from the heart.

"I think one of the things I'd say about Michael from throughout his career is that people who worked with him never had a bad word to say about him," the Briton said in a Formula One podcast.

"Within the teams he worked in, he always created a great impression and a great loyalty."

Schumacher spent 18 seasons, over a 21-year period, in Formula One and took at least one win in 15 successive campaigns (1992-2006).

Hamilton has so far done that for 12 in a row.

The German's career was also clouded by controversy, his achievements countered by the feeling that he benefited from the best car and a subservient team mate throughout his Ferrari years.

But his talent was indisputable, particularly in wet conditions, and his mind-management and ability to gel a team around him set him apart.

He was also a family man, fiercely guarding his privacy away from the track.

Son Mick, 19, is now set to debut in Formula Two, the rung below Formula One, this year after winning the European F3 title while 21-year-old daughter Gina-Marie is a rising equestrian talent.

"Happy Birthday to the best dad," she said in an Instagram post accompanying old photographs of her smiling father in racing overalls.