The Cecil Afrika-inspired Blitzboks lead 14-0 at halftime.
The South Africans cantered in for three more converted tries in the second half, while Japan closed out the scoring in the match with an unconverted try in the final minute.
Siviwe Soyizwapi, who scored against Scotland, dotted down twice against the Japanese, with Werner Kok and Afrika (2) also crossing the whitewash.
Later on Saturday, the Blitzboks round off their day-one action with their remaining pool game against Canada (6.42pm). It is a must-win game if coach Neil Powell and his team want to qualify for the Cup quarter-finals on Sunday.
The Blitzboks could win the World Series title if they win the Paris Sevens.
African News Agency (ANA)