The pair put on an unbeaten 63 for the seventh wicket with Renshaw four runs short of a fifty and Cummins 33 not out.
The umpires took the players off one hour before the scheduled close as windy and gloomy conditions coincided with South Africa bringing back the pacy Anrich Nortje into the attack and when he sent a bouncer over Renshaw’s head, play was immediately halted after the light meters were consulted.
The umpires waited out in the middle for another 45 minutes before calling off the day's proceedings.
Nortje was returning after almost four years away from first-class cricket, and took two wickets in the space of three balls before tea to give the hosts the upper hand.
The 32-year-old had not played since March 2023 as persistent injury and a focus on the T20 format kept him away from red-ball cricket but he justified his recall with a fiery spell after lunch that left the tourists teetering.
South Africa captain Temba Bavuma predicted Nortje could provide “X-factor”, and he did as much in having Alex Carey caught at second slip off a fuller delivery that clocked in at 143 kph and then, two balls later, snagging Beau Webster caught behind to see Australia fall to 124-6.
"To be honest I thought I wasn't going to get a wicket for the next few weeks the way the ball was passing the bat but then it was good to take the wickets and get the blood flowing again," Nortje said after the day's play.
The fortunes of Nortje, who finished the day with 2-40 off 13.2 overs, contrasted with those of Australian cancer survivor Nic Maddinson, also making a comeback, but who failed to get a run.
The 34-year-old, a surprise choice to open after Australia won the toss and chose to bat, survived 33 balls without scoring before Marco Jansen bowled him to end a remarkable return following his battle with cancer last year.
His duck was the longest by an Australian opener in test history and the third-longest by any test opener.
It was Maddinson’s first test appearance in almost a decade, having not played first-class cricket for 18 months as he fought testicular cancer that had spread to his lungs. He was only named in the team half an hour before play started.
Australia counter-attacked after Travis Head (9) and Maddinson went early, with Marnus Labuschagne and Steve Smith putting together a 60-run partnership for the third wicket before Wiaan Mulder (2-31) trapped Labuschagne lbw for 37 with the last ball before lunch.
Smith scored 25 on his first test appearance in South Africa since the 2018 tour that led to his one-year ban for his role in the ball-tampering scandal known as "Sandpapergate".
Smith walked to the crease without any noticeable heckling from the sparse crowd and quickly set about speeding up the scoring, with two sixes when spinner Keshav Maharaj was introduced to the home attack.
However, Smith was dismissed in the first over after lunch, getting a thick edge to gully to earn Jansen (2-35) a second wicket.
"Some pace, some bounces, some nibble, some spin, but I think there were moments in the game where I felt like we were getting close to having the foot on their throat," Renshaw said.