Morkel strikes twice to leave Australia reeling after Du Plessis ton

Australia face an uphill battle to avoid defeat in the fourth Test against South Africa after they ended day four on 88-3 in pursuit of a monumental target of 612.

Captain Faf du Plessis hit his eighth Test century as the Proteas piled on the runs before eventually declaring their second innings on 344-6, leaving their opponents a nigh-on impossible task.

However, Cricket South Africa explained the reason behind the team choosing to bat on for so long on Twitter, revealing injury issues for Vernon Philander (groin) and Kagiso Rabada (stiff back) in addition to the side strain for Morne Morkel that was already common knowledge.

Still, the seam trio were all able to bowl in the final session of play as Australia's much-changed top order struggled to cope in difficult conditions.

Morkel, playing in his final Test, fought through the pain to trap openers Matt Renshaw (five) and Joe Burns (42) - both summoned at short notice in the fall-out from the ball-tampering scandal - lbw with deliveries that moved late in the air.

In between Usman Khawaja fell to Keshav Maharaj, umpire Nigel Llong judging the left-hander had failed to offer a shot to a delivery from the spinner that turned sharply out of the rough.

Peter Handscomb reached 23 not out and Shaun Marsh was unbeaten on seven at the close, with the tourists requiring the small matter of 524 runs in the three remaining sessions.

South Africa are the only side with a hope of victory now, though their banal efforts with the bat to set up a declaration suggest they will be happy enough with a 2-1 result at the end of a highly controversial series.

They had patiently piled on the runs after resuming on 134-3, Dean Elgar taking 65 balls to add to his overnight total of 39 as the scoring rate came to a standstill at times in the morning session.

After overcoming a nasty knock to the right index finger he fractured earlier in the year, Du Plessis reached his ton courtesy of a thick edge to the third man boundary.

He eventually departed for 120 when Pat Cummins had him well caught by Handscomb at second slip, ending a 170-run stand for the fourth wicket.

Elgar failed to follow his skipper in reaching three figures, falling for 81 when trying to hit Nathan Lyon over the top, while Cummins (4-58) trapped Quinton de Kock soon after to leave the score at 273-6.

Temba Bavuma and Philander were unbeaten on 35 and 33 respectively when Du Plessis finally put Australia out of their misery, allowing Morkel to shine in the gloom in his swansong appearance.