Colombo - Left-arm spinner Xavier Doherty grabbed three wickets and all-rounder Shane Watson took two as Australia kept South Africa down to 146-5 in the World Twenty20 in Colombo on Sunday.
The Proteas, who need a win to stay in contention for the semi-finals after losing to Pakistan on Friday, lost quick wickets after being sent in to bat in the Super Eights match.
JP Duminy's 30 was the best score by a top-order batsman as South Africa slipped to 86-5 before Farhaan Behardien and Robin Peterson added 60 from 35 balls for the unbroken sixth wicket.
Behardien made 31 not out and Peterson returned unbeaten on a brisk 32 off 19 balls, the pair taking 28 runs in the final two overs.
Australia, who beat India in their first group two match, will close in on a semi-final berth if they win.
Doherty, opening the bowling after replacing seamer Daniel Christian for the match, removed Richard Levi with the third delivery and sent back Jacques Kallis in his second over.
It became 33-3 in the sixth over when Watson struck fifth ball as Hashim Amla edged a leg-side catch to wicket-keeper Matthew Wade after making 17.
Doherty, returning for his second spell in the 11th over, had Duminy stumped and Watson dismissed skipper AB de Villiers to reduce the Proteas to 86-5 in the 15th over.
Australia: George Bailey (captain), David Warner, Shane Watson, Matthew Wade, Michael Hussey, Cameron White, Glenn Maxwell, Brad Hogg, Xavier Doherty, Mitchell Starc, Pat Cummins.
South Africa: AB de Villiers (captain), Hashim Amla, Farhaan Behardien, Johan Botha, JP Duminy, Jacques Kallis, Richard Levi, Morné Morkel, Wayne Parnell, Robin Peterson, Dale Steyn.
Umpires: Ian Gould (ENG) and Kumar Dharmasena (SRI)
TV umpire: Richard Kettleborough (ENG)
Match referee: Jeff Crowe (NZL)