South Africa's brittle batting line-up is struggling again as Australia move to within six wickets of their first Test series win against the Proteas at home in 17 years.
Australia have continued their utter domination of South Africa at the MCG, with the visitors stumbling to 4-120 at lunch on Thursday.
South Africa still require another 266 runs just to force Australia to bat for a second time.
The Proteas have failed to pass 200 during their past seven Test innings, a dramatic fall from grace for a team that boasted batting legends Hashim Amla, AB de Villiers and Faf du Plessis when they last faced Australia in 2018.
Starting day four at 1-15 after losing captain Dean Elgar late on Wednesday, South Africa were unable to build any serious momentum on a good batting wicket.
Experienced No.4 Temba Bavuma (37no) and wicketkeeper Kyle Verreynne (27), comfortably South Africa's best batter on this tour, are the unbeaten pair after putting on a 55-run stand.
Bavuma will need to go on and make a big score after being responsible for a calamitous run-out that resulted in Khaya Zondo losing his wicket for one.
Travis Head was quick to hit the stumps after Bavuma took off on an incredibly risky run, with Zondo caught well short of his ground.
Star Australia quick Mitchell Starc, who is bowling in serious discomfort with a finger injury, took the key wicket of opener Sarel Erwee (21).
A visibly sore Starc has bowled 13 overs for figures of 1-44 despite his injury putting him in doubt for next week's SCG Test.
The 32-year-old was involved in a tense exchange with Theunis de Bruyn, who he threatened to Mankad after South Africa's No.3 stood out of his crease.
"He's halfway down Punt Rd, wasn't he? It's bad enough in white-ball cricket, I don't know what the need is in red-ball cricket," Starc told host broadcaster Channel 7 during the drinks break.