FUJI, Japan, Sept 28 (Reuters) - Germany's Nico Rosberg will be demoted 10 places on Sunday's Japanese Grand Prix starting grid after his Williams team said they would have to change his car's engine.

Under Formula One rules, engines must last two consecutive races unless a driver fails to finish and the changed engine had done only one.

Rosberg, son of Finland's 1982 Formula One champion Keke, is hitting a rich vein of form, scoring points in his last four races.

The penalty will be a blow to Williams, who are fourth in the constructors' championship and 11 points behind Renault with three races remaining.

Rosberg was fifth fastest in Friday's first practice at Fuji.