Australia fast bowlers Mitchell Starc and Josh Hazlewood took two wickets each to rip through South Africa's top order in the second World Cup semi-final on Thursday.
Left-arm quick Starc sent back skipper Temba Bavuma for a fourth-ball duck in the first over and soon Hazlewood had the in-form Quinton de Kock caught for three.
The new-ball bowlers kept up the attack with the wickets of Aiden Markram (10) and Heinrich Klaasen (10) to reduce South Africa to 24-4 after the Proteas elected to bat under cloudy skies and rain in the air.
South Africa are chasing their first ever World Cup final after faltering in four semi-finals including two against Australia in 1999 and 2007.
The winners of Thursday's game will take on undefeated hosts India, who knocked out New Zealand in the first semi-final, in Sunday's final at Ahmedabad.