They return to the Grenada National Stadium to face South Africa on Saturday ahead of their final Super Eight game against Australia next Friday.

One more victory will assure New Zealand, who've never won the ICC CWC, of a semi-final spot.

"I suppose the toughest test is coming up now. People perceive these three teams coming up as semi-finalists.

"We know it's going to be difficult. We're almost guaranteed a semi-final spot but we can control our own destiny if we win tomorrow," he said.

"That's what we want to do and put all doubts away and then find a comfortable semi-final if there is such a thing.

"I think we're lucky the three teams we are playing are three very good sides and tipped to do well in this ICC CWC," said Vettori, who now needs just one more wicket to become only the third New Zealander to take 200 in one-day internationals after Chris Harris (203) and Chris Cairns (203).

"We know if we are going to win the tournament we have to beat one of these three teams in either the semi-final or the final. It's a good wake-up call in some ways."

New Zealand are now just one short of equalling their 10-match winning run of 2004, having already beaten Australia three times at home before coming to the Caribbean.

But they know they face a stern challenge in a Sri Lanka side that earlier this year squared both a Test and one-day series in New Zealand.

"They are a very strong team. We've always had a feeling that when sub-continental teams come to New Zealand they tend to struggle but Sri Lanka certainly didn't the last time they came out and, if anything, probably got the better of us in our home conditions," Vettori explained.

"That doesn't happen too often and we realise that if they were doing that in our home conditions, they are going to be tough in these sorts of scenarios which tend to suit them."

Vettori, who has taken 12 wickets at this ICC CWC at an average of 20.91, said New Zealand had still to decide whether to go in with the same side that beat Ireland by 129 runs in Guyana on Monday.

Both batsman Ross Taylor hasn't played since a group game against Kenya (because of a hamstring injury) and fast bowler Mark Gillespie, restricted to a substitute fielder appearance against Bangladesh because of an injury (of his shoulder) are available for selection.

But fast bowler Michael Mason, who injured his calf bowling against Bangladesh appears to need more time.

"We played the second spinner (Jeetan Patel) thinking Guyana would be low and slow and turn a bit, which it did," added Vettori.

"What we saw yesterday (in Grenada, when South Africa beat West Indies by 67 runs on Tuesday) there weren't too many spinners on show. It's hard to get a gauge on whether it will turn too much.

"We think it's going to be another low, slow one."