Johann Zarco is the 2015 Moto2 world champion on Sunday after injured rival Tito Rabat withdrew from the Japanese event.
Reigning champion Rabat fell while training at Almeria, undergoing surgery on Monday for a fracture to the radius bone in his left arm.
The Marc VDS rider still hoped to race in Japan but managed just eight laps in FP1, when he was slowest and last, and the team declared he would take no further part in the weekend.
Rabat is already 78 points behind Zarco, who only needs a 75-point advantage to seal his first world title at the end of Sunday's Motegi race.
“There was little pain, but I just didn't have the power in the injured arm needed to cope with the hard braking areas here at Motegi," Rabat said. "To continue this weekend would have made no sense.
"The championship is gone and second place is not worth the risk of crashing again and making the injury worse.
"I am disappointed we weren't able to fight to the end, but congratulations to Johann Zarco on the championship. He's ridden a faultless season and will be a worthy champion.”
Zarco is the first Frenchman to win the intermediate 250cc/Moto2 crown since Olivier Jacque in 2000.
The Ajo star has taken six wins this season, finishing off the podium just twice and will remain in Moto2 to defend his crown next season. Rabat meanwhile will step up to MotoGP with Marc VDS in 2016.
A decision is yet to be made regarding Rabat's participation in next weekend's Phillip Island. Tito is currently 22 points ahead of third place Alex Rins, with three rounds remaining after Motegi.