With Japan's famous win over Ireland on Saturday they have set up a possible quarterfinal matchup with South Africa, and the Springboks are wary of how dangerous the home team can be to their World Cup title chances.
Speaking after his team cruised to a 54-point win over Namibia in Toyota on Saturday, South Africa coach Rassie Erasmus said he was not at all surprised the Brave Blossoms might be in a position to win their pool and be his team's first opponent of the knockout stage.
"I always knew Japan was a realistic contender for going to the quarterfinals," said Erasmus.
"If they beat Scotland now they are the top of the pool, and if we manage to beat Italy then that is a realistic option, and to be honest a bit of a scary option."
Japan has to win Pool A to set up a possible meeting with South Africa, who by token of their first-up loss to New Zealand have to beat Italy and Canada to secure second in their pool.
"A week ago I said if we manage to beat Italy (and) if Ireland beat Japan then we might meet Ireland, but both of us have to get past those hurdles first," said Erasmus.
Erasmus blamed the humidity in Japan for his team's at times error-riddled performance against Namibia, and he pointed out that the Brave Blossoms' familiarity with the conditions has played into their hands.
"You could see it even in the Ireland-Japan game, the Japanese did really well, they are used to these conditions to really handle the ball up until the end," he said.
Although South Africa famously lost to Japan at Rugby World Cup 2015 in the "Brighton Miracle," they beat Japan earlier this month in the lead-up to this World Cup, but Springboks hooker-cum-No. 8 Schalk Brits said nobody should read too much into his team's 41-7 win in Kumagaya.
"I know it will be a different outing when we play them again," said Brits, who captained the Springboks against Namibia in the relatively unfamiliar No. 8 jersey.
"It's an unbelievable win by the Japanese side to beat Ireland today."
He said given the different conditions, stadium and stakes, the earlier result means very little.
"As you saw today by their execution of their game plan, their set piece was very good and they beat a great Ireland side."
"They pick clever players, they've got an unbelievable skill set and a work rate of note. So, like Rassie said, in the previous game the score was not a reflection of the game and it will be a different challenge."