Retired double MotoGP champion Casey Stoner suffered a big accident while leading Sunday's Suzuka 8 Hours endurance race.

The 29-year-old Australian took over the #634 Musashi Honda from team-mate Takumi Takahashi just before the end of the first hour, moving into the lead when the Factory Yamaha of Katsuyuki Nakasuga pitted soon after.

Taking part in his first motorcycle race since Valencia 2012, disaster struck for Stoner on the fast, curving, downhill approach to the hairpin. After a front-end wobble, Stoner ran onto the grass where he fell from his CBR1000RR – sending rider and bike somersaulting.

Stoner had been able to crawl away, but the Musashi team's hopes of a third straight Suzuka win were well and truly over. A safety car was needed to remove Stoner's battered Honda from the middle of the track.

Australian Josh Hook (TSR Honda) then led the race after the restart, before being passed by Nakasuga's team-mate Bradley Smith.

Later reports are that Stoner has suffered a broken Collar bone and a fractured left tibia.