PHOTOS: Morkel keeps SA on course for big win over wilting Australia

IMAGES from Day 4 of the fourth Test between South Africa and Australia in Johannesburg on Monday

• Scorecard

Tall seamer Morne Morkel took two wickets to restrict struggling Australia to 88 for three at the close on the fourth day of the final Test as South Africa marched towards an emphatic victory on Monday.

Having set Australia a massive 612 for a win that would level the series, the home side removed the tourists’ top three batsmen, despite injury concerns over their pace trio which delayed South Africa's declaration until the tea interval.

Peter Handscomb (23) and Shaun Marsh (seven) will set about saving the Test on the fifth day, but only a near impossible victory would ensure Australia avoid a first series defeat in South Africa for almost 50 years.

The home side have injury worries over all their fast bowlers, though, with Kagiso Rabada suffering from a stiff lower back, Morkel struggling with a side strain and Vernon Philander nursing a groin problem.

That meant they limited the number of overs the trio would have to bowl as the hosts declared on 344 for six after a stylish century from captain Faf du Plessis.

The wilting Australian side have been rocked by the ball-tampering scandal in which former captain Steve Smith, his deputy David Warner and batsman Cameron Bancroft were sent home after the third Test in Cape Town.

Having been dropped in the gully by Temba Bavuma off Philander, Australia opener Matt Renshaw’s tortured stay at the crease was ended by Morkel, who is playing in his final Test before international retirement, as he was trapped plumb lbw after a 42-ball innings that yielded five runs.

South Africa’s second wicket was more controversial as Usman Khawaja (seven) was given out lbw by umpire Nigel Llong off the bowling of spinner Keshav Maharaj.

The ball struck the left-hander outside the line of off-stump, but Llong and third umpire Richard Illingworth agreed he had not been offering a shot, a decision the Australian clearly felt was wrong.

Opener Joe Burns (42) had looked in good touch until he became the third batsman trapped in front, this time from a rare Morkel yorker that was hitting middle stump.

A relieved Du Plessis (120) reached his eighth Test century after passing 50 for the first time in 11 innings, seven of which have been single-figure scores.

He was out edging seamer Pat Cummins (4-58) to a diving Handscomb at second slip, ending a fourth-wicket partnership of 170 with opener Dean Elgar (81).

Elgar’s 250-ball vigil ended shortly afterwards when he tried to attack spinner Nathan Lyon (2-116), but skied his shot to Shaun Marsh at mid-on.

Quinton de Kock (four) was trapped lbw by Cummins, who recorded the best Test match figures of his career, 9-141.