Tokyo - South Africa are New Zealand's toughest opponents, but always good for a chat after the game, according to assistant All Blacks coach Ian Foster.

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The rivalry between the two teams goes back to 1921, when New Zealand beat the visiting Springboks 13-5 in Dunedin in their inaugural Test match.

Hiatuses during the apartheid era notwithstanding, the two sides have faced off 98 times, the All Blacks winning 58 times to the Boks' 36, with four draws.

"We've always had a huge regard for South Africans as a rugby team," said Foster ahead of the two teams' blockbuster Pool B match at Yokohama on Saturday.

"They're a team which we've probably had our most physical contests with and probably have the best chat in the shed afterwards with.

"There's always been a deep respect based on the level of commitment that both teams put into it."

Foster, who was also assistant to Steve Hansen when New Zealand defended their World Cup title four years ago, added that the All Blacks were always at a "heightened state" when they played South Africa.

"I know it's the World Cup and the World Cup's big, but our focus has been on this game for some time," he said.

"We're hugely excited. There's no better start than this one."

Foster pinpointed the All Blacks' increased "concentration levels" as a key factor in the game.

"Everyone's excited about doing the hard work behind the scenes, part of that preparation is we go into the game and we stay as calculated and cool as we can possibly be.

"It's an important game - you want to get your performance right. We want to go and kick off this tournament on a level (where) we want to really put marker down and say that's where we're at, for our own information so we can improve.

"There's been a lot of talk about South Africa, and they deserve that, but our focus is how good can we be."

But Foster wouldn't be drawn on what game plan he thought South Africa might employ.

"We can only guess what the South Africans are going to do," he said. "What we do know is they're a top-quality side.

"We're not thinking too far ahead in this tournament because this weekend is a pretty big weekend.

"As coaches, it's pretty good because we can put all our energy into that first game and whichever way it goes we can sort out our plan to follow up."

Foster also played down any concerns New Zealand might have at coming into this September 20-November 3 tournament after losing their long grip on international rugby's number one ranking.

"It doesn't make the slightest difference to us, quite frankly, because with regards to the ranking, we've got our own expectations," said the former Waikato back.