After taking two wickets on a rain-marred opening day of the second Test, Boland finished another wet Monday with figures of 4-31 at St George's Park.
But the stand-out moment of the shortened day came in the dying overs, when Renshaw somehow latched on to a Ryan Rickelton edge to his right at first slip.
With Rickelton (31) and Temba Bavuma (41no) having steadied the ship from 4-40, Renshaw stuck out his right hand and took the ball low to the ground off Boland.
The look of shock on the Queenslander's face summed up his surprise, and carried on a brilliant fortnight in the field for Australia following last week's win in Durban.
The grab also saved the blushes of Marnus Labuschagne who had put Rickelton down off Cummins two overs earlier on 26 at leg slip, marking Australia's first drop of the series.
Boland, meanwhile, was again superb after rain stopped play from starting until the final session of Monday.
Brought in for Nathan Lyon as Australia went with four quicks, Boland's career mark now stands at 86 wickets at 18.08 with an third five-wicket haul in sight.
After taking two wickets during a brief opening-day burst, Boland then struck in his second over of day two when he had Marques Ackerman caught behind.
Ackerman's shot was not a great one, with the debutant driving at a ball outside off.
But Matthew Breetzke's for the next dismissal was worse, when he pushed at a ball well away from off stump off Starc and edged it through to Alex Carey.
Bavuma has held the innings together for the Proteas in a real captain's knock, facing 108 balls for his 41 and playing a couple of square drives off Starc and Boland.
The shot of the day then came when he hooked Pat Cummins for a big six, which bounced through the gates and stopped traffic on the road outside St George's Park.
Australia have been without Josh Hazlewood on day two, who remained off the field with illness.
The tourists are hopeful he will be back on the field on Tuesday, but he may need to go at least a session without bowling after spending time off.
The 30.2 overs of cricket came after heavy overnight showers wreaked havoc, with the rain stopped by 10:30am but a wet outfield delaying play for a further five hours.
A win at St George's Park would wrap up the three-Test series for Australia after the 214-run victory in Durban last week, and make them the first visiting team to triumph in South Africa since England in 2019-20.
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