Marsh brought up his third test 50 in 125 balls and is now top scorer for the Australians in their first innings as he weathered the second new ball with a circumspect approach.
He and Tim Paine resumed on Friday with Australia on 225-5 but once South Africa took the second new ball, it took just four deliveries for Paine to edge a full delivery from Kagiso Rabada behind to wicketkeeper Quinton de Kock for 25.
Pat Cummins batted patiently before being undone by the spin of Keshav Maharaj, who bowled him through the gate for three, leaving Australia 251-7.
But Starc scuppered home hopes of restricting Australia to under 300 on a slow, batting-friendly wicket, hitting some lusty blows for a rapidfire 35 runs off 25 balls before Maharaj bowled him in the last over before lunch.
Maharaj now has four wickets for 103 runs off 30.4 overs and will resume at the start of the second session as South Africa look to finish off the tail.
Captain Steve Smith (56) and David Warner (51) contributed runs on the opening day with Vernon Philander taking two wickets for the home team.
Warner said after the first day's play that any total over 300 would be considered "a crucial lead".
(Reporting by Mark Gleeson in Cape Town; Editing by Pritha Sarkar)
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