An injury to key New Zealand batsman Ross Taylor late on day two has cast a shadow over the home side's progress as it looks to chip away at South Africa's first-innings total of 308.
New Zealand was well-placed at 177 for three at stumps but Taylor's injury is a major setback.
He pulled up lame with a calf injury and needed assistance leaving the playing surface at the University Oval at about 5.30pm.
It is unclear whether he will play any further part in the game, although team-mate Trent Boult did not seem that optimistic.
''The way he walked off, it doesn't look too good but we'll see how he pulls up [today].''
Taylor will be monitored overnight and will have a scan as soon as possible.
It has been a rough few days for Taylor. He batted in a black armband yesterday after a good friend's brother was killed in a car accident on Wednesday, Taylor's 33rd birthday.
Despite the uncertainty surrounding Taylor's return, Boult felt the match was evenly poised.
''I think we fought very hard with the ball [yesterday] and got a lot of reward for keeping it very simple - and not letting them get too far ahead of the game.
''In terms of the bat it is nice to build a couple of partnerships - and to be 120, 130 behind at the end of day two- it is nicely balanced.''
Shortly after Taylor was forced to retire hurt on eight, Kane Williamson survived a run out thanks to a desperate dive.
That would have been a crushing blow for the Black Caps and the crowd of just over 2200.
Williamson is undefeated on 78 and is the key to the Black Caps' prospects today, particularly if Taylor does not return.
Williamson needed a lot of luck to be not out at stumps. He edged the ball three times only for it to fall just short of the slips each time.
But he also mixed in some glorious shots which we have come to expect from the classy right-hander, and his battle with Vernon Philander is absorbing.
Williamson arrived at the wicket with his side 15 for one after Tom Latham chased a delivery and nicked out for 10.
He combined with rookie international opener Jeet Raval in a New Zealand record second-wicket stand against South Africa of 102, eclipsing the old mark of just 90 set by Nathan Astle and Matt Horne in 1998-99.
Raval brought up his half-century with a sparkling shot down the ground, but had a lapse of concentration of 52 and chipped the ball to midwicket.
Henry Nicholls' innings was snuffed out by a sensational catch from Hashim Amla.
He tumbled forward and intercepted the ball just above the grass top.
Left-arm spinner Keshav Maharaj claimed both scalps.
Earlier, the Black Caps finally managed to pry opener Dean Elgar from the crease for a career-best 140.
The tenacious left-hander batted for nearly seven hours before he got an edge to a cracking bouncer from Neil Wagner.
Quinton de Kock's arrival at the crease was countered with Jeetan Patel's reintroduction. He nabbed the dangerous left-hander's wicket twice during the one-day series and the home side fancied a repeat performance.
Patel went to work, lobbing the ball up outside off. De Kock walked right into the trap, slicing a drive to Wagner, who dived forward and took a good catch at backward point to dismiss the batsman for 10.
Temba Bavuma played a valuable hand for the visiting side with 64, and Philander resisted with 21 but South Africa lost its last five wickets for 56 runs.
