The South African cricketers are 404 for three, a lead of 195 over Australia after two days of the third Test in Cape Town.
Ashwell Prince scored 150, while captain Jacques Kallis is 102 not out and AB De Villiers is on 39.
Prince demoralised debutant leg-spinner Bryce McGain whose Ashes hopes suffered a blow as he conceded the ugly figures of none for 102 from 11 overs.
Middle-order batsman Prince, who was reluctant to take on the role of opener in his comeback match, thumped 19 fours and two sixes in a brilliant six-hour innings.
Australia have already won the series.
