After a delayed start due to a wet ground, New Zealand took the wicket of first-day centurion Hashim Amla, who added just four to his overnight total to go for 110, but were otherwise frustrated as Du Plessis worked towards his own ton.
Together with Dean Elgar, he helped South Africa to 390 for five at the interval.
Du Plessis' progress will be a particular source of angst for New Zealand, who missed the chance to remove him on 42 on Friday when replays showed he gloved a delivery to wicketkeeper BJ Watling, only for the Black Caps to decide against referring the decision.
He could not bring up three figures before the late lunch, however, with Colin Munro bowling a maiden in the final over of the session.
Amla had been in imperious form on day one but soon succumbed on Saturday morning, edging a Trent Boult delivery behind down the leg side.
New Zealand spent the rest of the session threatening Du Plessis and Elgar with some decent bowling, but did not find much reward.
- Sports & Recreation
- Dean Elgar
- New Zealand
- Hashim Amla