Faf du Plessis made his first major contribution of the series as South Africa moved further ahead of Australia on the fourth day of the fourth Test at the Wanderers Stadium on Monday.
South Africa were 202 for three at lunch, an overall lead of 469. South African captain Du Plessis made 81 not out and opening batsman Dean Elgar was on 59 not out.
Du Plessis, who made only 55 runs in his first seven innings in the series, needed lengthy treatment after a ball from Pat Cummins split open the right index finger he broke earlier in the season. He suffered the blow when he was on 43.
Only 20 overs were bowled on a heavily overcast morning. Bad light delayed the start, there was a 16-minute stoppage for more bad light and a further delay when Du Plessis was injured.
South Africa, who lead the series 2-1 and need only to draw to clinch their first home series win against Australia since 1969/70, added 68 runs without losing a wicket.
Elgar crawled to a half-century off 199 balls, ironically reaching the mark with a six off Mitchell Marsh.