Alviro Peterson was the man out for 22, but captain Graeme Smith was unbeaten on 40 at the interval having been joined in a stand of 70 by Hashim Amla (32 not out).
South Africa won the first Test by an innings inside three days.
Petersen inside-edged the first ball of the match, by Trent Boult, for a lucky four before Doug Bracewell struck Smith a nasty blow on the head in the fourth over.
Petersen took another boundary off each bowler before being dismissed by Bracewell, top-edging a weak hook to Jeetan Patel at fine leg.
Amla got away with a leading edge off his first delivery but Smith got into gear with boundaries in successive Neil Wagner overs.
A swept boundary by Amla brought up the Proteas' half-century but Smith got away with a couple of edges as the bowlers found enough swing to provide encouragement.
Yet another, off Boult, fell short of second slip but was sandwiched by boundaries from each batsman, and lunch arrived with the hosts just one run short of three figures.
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