Missing a string of experienced players and still trying to shake off the unsettling exit of Ross Taylor as captain, New Zealand produced a gutsy fightback following a sobering eight-wicket loss in the tour-opening international two days ago.
“Martin Guptill's innings was top drawer,” new skipper McCullum said. “A phenomenal effort from a guy who has hardly left his room in the last couple of days.”
Boosted by Davids' rapid start and the in-form Du Plessis' 63 from 43 balls, South Africa was going strongly until the floodlight failure with four overs of its innings to go.
After a lengthy delay, the teams returned with umpires opting to play on in diminished light and Doug Bracewell (3-33) took three wickets in the final over of the Proteas' innings to begin New Zealand's revival.
Guptill started cautiously and earned some late luck when he was caught off a no ball and then dropped on 89 as Robin Peterson couldn't hold onto a high, swirling catch.
The right-hander had accelerated when New Zealand needed it, though, eventually hitting nine fours and his six maximums in his man-of-the-match-winning performance.
“We didn't play a bad game, we were just outplayed by a great knock,” South Africa captain Du Plessis said.
The teams wrap up the T20 series next week before two tests and three one-day internationals.
