Skipper Pat Cummins bowled Australia into contention to pull off a remarkable victory with a fiery spell against South Africa in an absorbing third Sydney Test on Saturday.

Cummins captured three for 29 in a hostile 14 overs to set up the Australians for a dramatic final day charge for victory and a series clean sweep against the hapless Proteas.

At the close on day four, South Africa were batting for survival at 149 for six in reply to Australia's 475-4 declared with Marco Jansen on 10 and Simon Harmer six.

The outgunned South Africans have only passed 200 once so far in this series.

The Australians need 14 more wickets to pull off an astonishing victory and cannot be counted out after rocking the tourists in Saturday's two dominant sessions of the rain-marred Test.

Josh Hazlewood made the initial breakthrough, removing Dean Elgar for 15 continuing the Proteas' skipper's wretched series of low scores.

Hazlewood, bowling a lively opening spell, got Elgar with a brutal short-pitched delivery which clipped his glove for a catch by wicketkeeper Alex Carey.

Nathan Lyon stunned Sarel Elwee who did not play a shot and was bowled top of off-stump for 18 leaving the tourists 37-2 in the 18th over.

Four balls later Cummins claimed his first victim with Heinrich Klaasen, caught off a lifter leg-side by Carey for two.

Hazlewood broke through again after tea with an inswinger that kissed the edge of Temba Bavuma's bat for Carey to do the rest.

Bavuma clouted two sixes off Lyon in his 35 off 74 balls but was a big wicket for the Aussies to leave the visitors 85-4 in the 39th over.

"The boys said it's starting to reverse swing so if we can keep the ball dry and put a bit of work into it, then hopefully the boys can do the job with reverse swing," Lyon said during a drinks break.

"There is a bit of rough out here now so we'll try to get them on the front foot and see what we can do."

Cummins removed Khaya Zondo with a superb yorker that trapped him leg before wicket for 39 and South Africa's fifth wicket.

The skipper charged in to lead the Australian bowling assault with two fearsome deliveries thudding into Marco Jansen's upper body.

Cummins quickly got another wicket, with Kyle Verreynne edging to Steve Smith at slip for 19 for Smith's 151st Test catch.

Khawaja misses double ton
After rain again prevented play in the morning session, Cummins declared his team's first innings after lunch to chase victory.

Cummins's decision to declare deprived opener Usman Khawaja of a first Test double century, with the elegant opener stranded on 195 not out.

The weather forecast is for improving conditions and Australia have potentially 98 more overs on Sunday's final day to finish off the South Africans.

In the first Brisbane Test, the Proteas lost all 20 wickets in 86 overs and in Melbourne South Africa lasted 137.3 overs.

Australia are bidding for a series clean sweep to seal a place in the World Test Championship final in London in June.

Australia are 2-0 up in the series after winning the opening Test by a six-wicket rout in Brisbane inside two days, then hammering the Proteas by an innings and 182 runs in Melbourne.