It was a fine hand, taking the heat off de Villiers. With a distinctive technique in which he often stepped well across his stumps, exposing them behind his legs, du Plessis took 14 in three balls from seamer Doug Bracewell, kept the bowlers on the back foot, and was positivity personified.
"He was under a lot of pressure and he just played beautifully," de Villiers said of the 27-year-old from Pretoria. "I had to speak to him to calm him down but the way he struck the ball from the outset was brilliant."
Du Plessis is evidently given a free rein within the South African side to play his natural game. Saturday night was just his 19th ODI, so he's still bedding himself in, although he played all five in the recent series win over Sri Lanka at home.
New Zealand were left to rue a day on which they could not make sufficient headway against an impressive pace attack, and get into a position to attack later in the innings, having won the toss.
McCullum's determined 56 and Kane Williamson, with an intelligently crafted 55, did well. They were on the point of calling the batting power play when McCullum fell to a fine catch at deep point by Robin Peterson.
Jesse Ryder had no excuse for a thoughtless play shortly after. Twelve had been taken from the first five balls of a Jacques Kallis over when Ryder skied an attempted pull to be caught in the deep.
At 35 for three, New Zealand were right in the contest, but de Villiers steadily pulled the contest South Africa's way, his 106 not out coming at run-a-ball rate. Du Plessis' key hand did the rest and the momentum lies decisively with the tourists.
Some of the New Zealand fielding was ordinary late on, and the bowling wilted, too.
"De Villiers played an outstanding innings and we can learn a lot from that," Williamson said yesterday. "We're up against a very strong side and we need to be better to beat them."
Which may sound like stating the obvious, but New Zealand are being obliged to play near-faultless cricket in all departments.
The batsmen will continue to be asked tough questions by quicks Dale Steyn, with his late outswing and ability to skid short lifters onto the batsmen, and tall Morne Morkel, with his awkward bounce.
"Steyn's obviously a world-class bowler and a great challenge. Morkel bowls back of a length and troubles batsmen that way," Williamson said.
"They bring different challenges but it's exciting to play against guys like that."
