Black Caps captain Kane Williamson top scored with an unbeaten 106 runs.
Lockie Ferguson took down another splendid wicket that marks his third as of yet.
This encounter will be decided depending on how South Africa's slightly fragile batting deals with the New Zealand bowling unit.
Team New Zealand will look to maintain their unbeaten streak in the ICC Cricket World Cup 2019 when they take on South Africa at Birmingham on Wednesday.
South Africa, who suffered a heartbreaking loss to New Zealand in the last edition, had a dismal start to their World Cup campaign, losing three straight games followed by a wash-out before registering their maiden win, against bottom-placed Afghanistan.
Their circumspect approach on a tricky two-paced pitch was not quite enough though, with a fourth defeat in six matches effectively eliminating South Africa from semi-final contention.
In what has become a familiar refrain at global events, things might have been different had South Africa taken any of the half-chances towards the business end of their defence of a modest 242.
With 105 needed off 17.4 overs it required some de Grandhomme swipes to ease the mounting pressure and he delivered on familar turf where he's played for Warwickshire.
The match was reduced to 49-a-side over affair after wet ground outfield delayed the start.
While most of the cricketers rate their achievements for national team above that for T20 franchises, South Africa star Quinton de Kock is an exception. But also the innings that Kane played, a 100 off 130 balls, taking his team home, was too good for us on the day. They were then given a run for their money by Bangladesh in the next game but still succeeded in winning the game by two wickets.