South Africa openers Marques Ackerman and Aiden Markram cashed in on a benign Newlands surface on the first morning of the series-deciding third Test as a new-look Australia face a long haul in the field.

After South Africa were surprisingly sent in to bat, Ackerman and Markram batted through the opening session with relative ease under the Cape Town sunshine. Ackerman hit a rollicking maiden Test half-century and reached lunch on 82 off 75 balls with Markram playing the support act.

Australia's misery was complete just before lunch when Travis Head dropped a straightforward chance at cover to reprieve Markram on 39, leaving Mitchell Starc to bite his collar on the way back to his bowling mark.

Without dropped quick Scott Boland, Australia's attack failed to penetrate as South Africa scored at a run rate of 5.18. Quick Josh Hazlewood was the only bowler to have an economy of under five an over.

It's been a torrid start for Australia in their return to Cape Town since the sandpaper scandal in 2018. They might already be rueing skipper Pat Cummins' shock decision to bowl first after South Africa were barely made to sweat much to the delight of a near capacity crowd.

Australia were rocked before the match by Steven Smith being ruled out after he failed to recover from a neck injury sustained in their big second Test defeat. Smith had a brief net this morning while wearing a bandage or heat patch on his neck then wasn't seen for about half an hour.

He was part of mass changes with Cameron Green, Josh Inglis, Nathan Lyon and spinning allrounder Cooper Connolly recalled while Boland, Nic Maddinson and Beau Webster were dropped.

It was the first time Australia had made four changes since the Johannesburg Test in the aftermath of the sandpaper scandal.

Cummins became the first captain to elect to bowl at Newlands since Faf du Plessis in 2019 against Pakistan. He had hoped for early seam-friendly conditions, but the new-ball produced little threat against Markram and Ackerman, who were rubbing their hands together.

Markram has looked in sparkling touch through the series without making a really big score. He knew this was a golden opportunity and, after a couple of watchful overs, stroked two superb boundaries off Starc.

With little movement off the surface, Starc ramped up his pace to near 150 kph and almost had Ackerman caught off the glove at leg slip but the ball flew past a diving Lyon.

That was the only nervy moment for South Africa in the opening half-an-hour as Cummins stepped up determined to rectify the situation. But Markram counterattacked with a huge blow over midwicket after launching into an innocuous short delivery.

Cummins in the 11th over turned to Lyon, desperate to make an impact after making way for Boland in the second Test. But he was expensive with Ackerman's confidence rising after smashing a six down the ground and soon he raised his bat having notched his half-century off 62 balls.

Markram almost gifted his wicket when his loose cover drive was hit straight to Head, who dropped a sitter. Ackerman did not decide to shut up shop before the interval as he launched into Starc with four consecutive boundaries to punctuate South Africa's dominant session.