Marnus Labuschagne's Test career hangs by a thread after falling for a duck on the second morning of the series-deciding third Test as Australia's rejigged batting-order faced a thorough examination against South Africa offspinner Simon Harmer.

Openers Travis Head and Matt Renshaw had launched Australia's first innings with a brisk 58-run partnership on a sedate surface under the Cape Town sunshine.

But hopes of compiling Australia's first century opening partnership since the 2023 Ashes tour were dashed with Harmer's introduction in the 18th over. He had Head on 41 chopping onto his stumps in a familiar dismissal, leading to Labuschagne coming to the crease having been fortunate to survive the axe after a prolonged form slump.

Labuschagne lasted just five deliveries after poking Harmer to first slip, with scrutiny on his place in the team set to intensify.

Renshaw has dug in and looks focused on the long haul while new No.4 Cameron Green was assured before the lunch break in his return after missing the opening two Tests with an abdominal injury.

South Africa's seamers could not find movement and there was no hint of the menacing swing conjured by Mitchell Starc in his extraordinary six-wicket burst in the final session of day one. Quicks Kagiso Rabada and Marco Jansen failed to seriously threaten with the new ball and erred in bowling slightly too short. Harmer found occasional sharp turn and returned the figures of 2 for 5 off 4 overs as he single-handedly engineered a fightback.

After Starc's heroics late on day one limited South Africa to a modest first innings total of 329, all eyes were on Australia's new-look batting-order without Steven Smith who was ruled out with a neck injury sustained in the second Test.

Renshaw moved back to the top of the order, where he had batted in 20 of his 29 previous Test innings. He had enjoyed his shift to No.5 for this series, making 190 and 82 in a spectacular first Test performance, but once again partnered Head after replacing Nic Maddinson, who had an unsuccessful return to Test cricket.

Jansen entered on 99 Test wickets, but a milestone celebration was not forthcoming with his radar awry. He could not consistently hit the right length and was punished by Renshaw, who targeted his favoured leg side.

In trademark fashion, Head lived more dangerously and twice sliced boundaries just past backward point to leave South Africa frustrated.

They were the only nervy moments against the new ball although Renshaw wore a fierce 145 kph bumper from Rabada on his right shoulder, brushing off the incident with a beaming smile.

South Africa captain Temba Bavuma turned to speedster Gerald Coetzee, playing his first Test match since November 2024 after replacing injured quick Anrich Nortje.

Coetzee started with a 146 kph thunderbolt but inaccuracy proved costly against a rampant Head, who went on the attack before the drinks break. Head pounced on width and brought up the fifty partnership in style with superb improvisation to uppercut over the slips.

With his pace attack blunted, Bavuma turned to Harmer and it paid instant dividends with Head's decision to not let him settle backfiring. Labuschagne entered with his career in the balance although he had made starts through the series. He could not get off the mark and soon trudged off the field amid familiar disappointment.