Toyota City - South Africa coach Rassie Erasmus said Saturday Japan were a "scary" proposition after the hosts' staggering upset of Ireland put them on collision course with the Springboks in the Rugby World Cup quarter-finals.

South Africa famously went down to Japan in the 2015 World Cup's 'Miracle of Brighton', an upset the Brave Blossoms reprised with their 19-12 win over Ireland - who came into the tournament ranked world number one.

The Springboks beat Japan 41-7 in a World Cup warm-up but after the seismic shock in Shizuoka, Erasmus said he had always tipped them to get out of Pool A, where they have two wins from two and games to come against Samoa and Scotland.

"I always knew that Japan was a realistic contender for going through to the quarter-finals," Erasmus said, after South Africa ran through minnows Namibia 57-3 in a Pool B stroll.

"Probably if they beat Scotland now they can end top of the pool and if we manage to beat Italy then that's a realistic option. It's almost a bit of a scary option."

South Africa lost 23-13 to defending champions New Zealand in their opening game and now face a "crunch" clash against Six Nations side Italy on Friday, ahead of their final Pool B fixture with Canada.

On Saturday, Erasmus changed 13 players from the team that lost to the All Blacks, even deploying hooker Schalk Brits at number eight, but they still managed to run in nine tries against Namibia.

Namibia coach Phil Davies, however, who also fielded a much-changed line-up after their battling defeat to Italy, said his team was going in the right direction.

Namibia, still winless in 21 World Cup games, lost 142-0 to Australia in 2003 - the biggest score margin in the tournament's history - and went down 87-0 when they last played the Springboks in 2011.

"The scorelines would suggest we are improving. We played with a lot of courage tonight... overall we are progressing," Davies said.

"We played Italy last week and we were very competent in certain facets of the game as we were tonight. Our line-out, against arguably one of the best line-outs in the world, coped well.

"There are lots of positives and that's what we've got to take out of it and we've got to move forward and take on New Zealand next week."