Renshaw's 190 on Saturday was a masterclass in batting with the tail, as he spent the final four hours of Australia's innings controlling the strike.
It allowed the tourists to recover from 6-124 on Friday and 8-230 early on Saturday to reach 390 and take command of the Test after two days, with South Africa 1-27 in reply.
Renshaw was on 69 when Nathan Lyon walked to the wicket, before the pair's 134-run partnership was Australia's highest for the ninth wicket in 132 years.
Crucial in that was the way Renshaw maintained the strike, regularly turning down runs before taking singles late in the over the Proteas kept fielders back.
It allowed Renshaw to at one stage face the first ball of 13 straight overs, shielding Lyon and facing almost 70 per cent of the balls in the last two wickets.
"It just seemed like he got off strike so easily when he wanted to," Mulder, who took 4-88 for the Proteas, said.
"His approach was so different from the first four balls to the last two balls (of an over) which made it difficult to keep him on strike.
"But maybe that's where we could have been smarter with the field.
"Sometimes it felt a bit easy for me ... Maybe that's something to think about especially as the game progresses.
"If a batter is in and he's batting with a tail maybe we should make it a little bit harder for him to get off strike."
The other benefit of Australia's fightback through Renshaw, Pat Cummins and Lyon was the overs put into South Africa's quicks, as the Proteas taking six hours to claim the final four wickets.
Renshaw viewed that as important as scoring runs, with South Africa's two spinners largely ineffective and minimal gaps between Tests in this series.
Anrich Nortje (2-88) bowled 28 overs in his first first-class game since March 2023, Mulder 27 and Marco Jansen 31 for his 3-68.
Mulder conceded he did not believe Nortje may be able to back up for the next Test in Gqeberha, with spearhead quick Kagiso Rabada at least firming to play.
"Whether you'll see Anrich in the second Test I'm not sure," Mulder said.
"Someone like him who bowls express pace you don't necessarily want him to be playing all the test matches and bowling 27 or 30 overs in an innings.
"That's not (from) the selection panel or anything like that, I'm just saying what I'm thinking.
"If we bat well (the workloads) doesn't play such a big role (for this Test).
"As soon as you go over 20 overs that miles in the legs does impact you at some stage. But it's the first test so most of our bowlers are fresh."
20°C