Mitchell Starc took two wickets late in the first session as South Africa struggled to 63-3 at lunch Friday after winning the toss and batting in the crucial third test against Australia.
Shane Watson marked his return from injury by removing South Africa skipper Graeme Smith (16) before Starc bowled opener Alviro Peterson (30) and Jacques Kallis (2) in his last two overs before the interval.
Starc, who was 12th man for the drawn tests in Brisbane and Adelaide, led the new-look pace attack with 2-27 from eight overs.
Hashim Amla was not out 11 at lunch and AB de Villiers yet to score. The South Africans only need to draw the Perth test to retain the No. 1 test ranking, while Australia needs a win to replace them.