Only the best will do if the All Blacks want to maintain their momentum towards a successful defence of their Tri-Nations rugby title, skipper Richie McCaw says.

New Zealand meet South Africa in Cape Town on Saturday and a win, especially with a bonus point, would significantly advance their hopes of a fourth title in a row and ninth in total.

But McCaw said the tournament had shown in its last two matches how evenly matched the teams were and how just a small dip in performance could mean the difference between victory and defeat.

Two weeks ago in Auckland, in McCaw's return from injury, the All Blacks were superb in overwhelming Australia 39-10 in Auckland.

But just seven days earlier in Sydney, with the champion openside flanker watching on helpless from the stands, the virtual opposite occurred.

That time, it was the Wallabies running out comfortable 34-19 winners against an error-ridden New Zealand.

McCaw said the way the All Blacks had been able to bounce back from one of their poorer performances for a while had given plenty of satisfaction, but it also provided an illustration.

"Those two games in particular show there is very, very little between all three teams in the Tri-Nations," he said.

"If you are not 100 per cent and get things dead right, you will come second. You just have to make sure you get your preparation right to play as well as you can.

"The win two weeks ago was satisfying, but it's easy to do one. It's about being able to back that performance up and that's what we're trying to do."

While McCaw got plenty of kudos for his exceptional display at Eden Park despite six weeks on the sidelines, he said a loose forward relied on his front five to provide the platform.

"How a backrower operates comes off what happens in front of you," he said.

"The last match was a great indicator of that. The front five got stuck in and made our jobs a lot easier."

The loose forward battle shapes up as one of the intriguing features of the upcoming contest at Newlands, with McCaw, No 8 Rodney So'oialo and blindside flanker Jerome Kaino up against Juan Smith, Pierre Spies and Schalk Burger.

The destructive Burger is making his return to the Springbok team after having sat out the 63-9 win over Argentina in Johannesburg last weekend to allow him to get over a neck problem.

"He adds a lot of energy there, and in the physical side of things he is always in amongst it," McCaw said.

"How effective you are depends on whether you get front-foot ball or not. I know certainly, when you don't have front-foot ball, he can be a real menace."

A New Zealand victory at Newlands would keep their title hopes in their own hands ahead of their final Tri-Nations match, against Australia in Brisbane next month.

Asked how much another Tri-Nations crown would make up for last year's World Cup deep disappointment, McCaw's response was `probably not a lot".

"But for this All Black team, we want to make sure every time we go into a Tri-Nations or any test, we perform as well as we can and hopefully get the results we're after," he said.

"We have two games left and if we can win those two games, we give ourselves a chance."

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