Almost a decade after his last hundred as baby-faced 20-year-old, Renshaw's superb 190 led Australia from 6-124 to a total of 390 on day two in Durban.
He did so with the brilliant aid of Pat Cummins (43) and Nathan Lyon (27), producing Australia's best ninth-wicket partnership since 1894 with the latter as they added 134 together.
South Africa then went to stumps at 1-27 when bad light again stopped play, after Josh Hazlewood had Ryan Rickelton caught edging a drive to third slip for a duck.
But this was a Saturday that belonged to Renshaw, some 3567 days after his last Test hundred and having been dropped from the team three times since.
The Queenslander looks a far more complete player now, having matured and added extra gears to his batting after being recalled to open in Australia's last Test in August.
"I had the opportunity in 2023 and didn't take it. So you probably think you're never going to get this opportunity again," Renshaw said.
"All the stuff that I've done in the past 10 years has led me to this moment. I wouldn't be where I was without all the failures.
"We're quick to judge failures, but I think some of the best, or definitely some of the best learning has come from those failures ... It culminated in this."
After coach Andrew McDonald had spoken last week about the importance of lower-order runs and being willing to get into grind, Renshaw helped Australia deliver.
The Yorkshire-born product was on 69 when Lyon came to the crease, and from that point controlled the strike masterfully as South Africa were guilty of letting the game drift.
The 30-year-old joked he was fine with missing out on a double century because he "probably turned down 40 or 50 runs", all the while lecturing Lyon at times to keep the spinner in his bubble.
"I always wanted that second (Test century). I had the monkey off the back early with that first one," Renshaw said.
"But this one, all the stuff that have come in the last 10 years, all the hard work, all the failures, all the success. This was nice just for me to get.
"So when I hit the ball, I was screaming at it, it was just 'go, go, go'. It all just hit me at once."
So good was Renshaw's innings, Australia may have another selection headache in what they do with the left-hander.
Nic Maddinson's selection and 33-ball duck drew widespread criticism on Friday, with the call in turn bumping Renshaw down to No.5 after he impressed in the middle order in ODIs.
But the question now will be: If Renshaw is this settled in the middle order, do Australia want to move him back to open once Cameron Green or Josh Inglis returns?
He was busy against the spin on Friday, and hit most of his early boundaries against the quicks behind square in the first half of his innings.
After bringing up his half-century in the first over on Saturday, Renshaw got through a tough period with the second new ball just after Cummins fell.
But as the morning went on Renshaw became more comfortable, cover-driving and cutting Anrich Nortje for two boundaries in one over.
He then hit his 204th ball from Marco Jansen to the long-on boundary to bring up his century, prompting a long embrace with Lyon and meaningful celebration.
After lunch he started reverse-sweeping the spinners to the boundary and even uppercut Nortje for six.
Renshaw eventually edged spinner Simon Harmer to slip two balls after tea, but by that point his 397-ball stay had ensured Australia were well on top.
At the other end, Cummins, Mitchell Starc (6) Lyon and Hazlewood (5no) saw out 226 balls between them and lasted a combined 388 minutes alongside Renshaw.
It also helped pump overs into the Proteas' quicks, with Nortje (2-88), Jasen (3-68) and Wiaan Mulder (4-68) sending down 86 between them.
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