Japanese rider Daijiro Kato is fighting for his life after sustaining severe head, neck and chest injuries in a crash at the season-opening Japanese Grand Prix on Sunday.
The 26-year-old, who lost control of his Honda as he approached the chicane, was rushed by helicopter to an intensive care unit after medical staff battled to get his heart beating again.
His condition is 'life-threatening', according to race organisers at the Suzuka track.
"Only quick treatment from the Suzuka medical staff saved his life. His heart was beating when he arrived at the hospital, but he is in a coma..." medical director Claudio Macchiagodena said.
He said, "Doctors continued to try to revive him at the hospital but his condition is extremely serious.
"It is difficult to make a medical forecast. Riders often surprise us with quick recoveries. We hope he will show signs of improvement in the next 24 to 48 hours and we can be in a position to give you more news."
Kato, who won the 250cc world title in 2001, skidded off on the second lap while travelling at over 200kph.
Italian Marco Melandri had broken his ankle, nose and femur in another high-speed crash during practice on Friday.
MotoGP world champion Valentino Rossi, who completed a hat-trick of Suzuka victories on Sunday, had called the high-speed bend 'too dangerous'.
Race director Paul Butler said organisers have yet to determine the cause of the crash.
"We have reviewed the video evidence and frankly we are none the wiser as to what caused this very tragic accident. Our thoughts and prayers go to Daijiro and his family," he said.