Talk about a day of topsy following turvy, as New Zealand discovered to their chagrin at the Basin Reserve today.

There they were at lunch, probably munching happily as South Africa were reeling at 104 for six, still 164 adrift of New Zealand's first innings 268.

Two hours later, after a session which produced 114 for none, the balance had altered noticeably and by stumps, South Africa had their noses in front by what might become a significant margin.

They will start the third day tomorrow at 349 for nine, 81 ahead, and feeling that a decisive shift may have taken place in the match, possibly the series.

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Remarkably New Zealand have only ever overhauled a larger first innings deficit and gone on to win a test twice - 144 against Pakistan in Christchurch 1994, and 134 against Sri Lanka at the Basin in 2015.

South Africa have scored 255 for their last four wickets in 65 overs. The last pair, Vernon Philander and Morne Morkel have put on 47 - equalling their best tenth-wicket stand against New Zealand set 55 years ago - and tall seamer Morkel has survived a solid clunk on the front of his helmet form a Tim Southee bouncer.

Still, the pitch is good and its reputation is of getting better over the last days of a test, not breaking up. New Zealand need to bat well tomorrow, but conditions will be a lot better than the first day.

"As conditions flattened out they made the most of it and played well," New Zealand seamer Neil Wagner said.

"After losing the toss and being put in, we're pretty happy with the position we're in. There's still a lot of cricket left to play."

New Zealand were primarily undone by a combination of being unable to sustain a strong morning session's work and some fine, resourceful batting by little Temba Bavuma and wicketkeeper Quinton de Kock.

Their stand of 160 is the second highest for South Africa's seventh wicket against New Zealand.

Where Bavuma dug in and was patient, de Kock went on the offensive.

He turned the game back on New Zealand in the hour after lunch, which ended with four boundary riders for both Wagner and Tim Southee as de Kock, whose instincts are invariably positive, took charge.

New Zealand might argue those outfielders are catching contenders against a hitter like de Kock; but anyone lobbing in from Iceland would have assumed the wrong team was on top at that point.

Try this for a day: New Zealand took four for 80 in the first session, quality bowling helped by some slipshod batting; South Africa totally dominated the second as New Zealand went through a head-scratching period, funky fields trying to find the right key to separate the pair; then three wickets were chipped away before Philander and Morkel threw in a few sharp jabs in the final half hour.

De Kock, dismissed by offspinner Jeetan Patel in their four innings against each other on this tour, got through that challenge and his offensive approach was just the ticket for South Africa in the afternoon.

He arrived at 94 for six and didn't get bogged down in a grafting battle. As his captain Faf du Plessis said the day before the test, de Kock has a simple, uncomplicated approach to the game.

He forced New Zealand to dance to his tune through the afternoon, the bowling got sloppy and short and that played to his strengths. New Zealand can rue a lost chance, but sometimes you simply have to give credit where it's due.