TOKYO — When the overlords of rugby a decade ago chose Japan to host this year’s Rugby World Cup, they hoped to spread the gospel of the sport beyond its traditional strongholds in Europe, Australia, South Africa and Argentina. A boost in participation in Asia, and maybe a few new sponsors and heightened interest from television broadcasters, seemed like realistic goals.

But few could have anticipated the roaring success of Japan’s team, which could rejuvenate the sport in a country where rugby has largely been limited to universities and old boys’ clubs. With victories over Ireland, Scotland, Samoa and Russia in the group stage, Japan became the first nation from Asia to win its group at a World Cup. The capstone was a defeat of Scotland on Sunday that sent Japan into the quarterfinal round for the first time.

There, Japan will face an even more formidable foe in South Africa on Sunday. The Brave Blossoms, as the Japanese team is called, will be tested like never before, and not only because the Springboks will want to avenge their shocking loss to Japan in the last Rugby World Cup four years ago. South Africa pummeled Japan, 41-7, in September in a warm-up match for this year’s tournament.