Almost a decade after his last hundred as baby-faced 20-year-old, Renshaw enjoyed the finest day of his career as his 190 led Australia to 390 on day two in Durban.
Josh Hazlewood then had Ryan Rickleton caught edging a drive to third slip for a duck, before South Africa were 1-27 when bad light stopped play.
Andrew McDonald had spoken in the lead up this Kingsmead Test about the importance of lower-order runs and being willing to get into grind, and Australia delivered on both on Saturday.
After Australia slumped to 6-124 midway through the first day of the series, Renshaw and Pat Cummins (43) combined for an 84-run seventh-wicket stand.
Renshaw and Nathan Lyon (27) then added 134 together, marking Australia's largest ninth-wicket partnership since 1894.
All the while Renshaw managed the innings magnificently, knocking back singles and keeping the strike with South Africa were guilty of letting the game drift.
Renshaw's last Test century came with in his fourth Test in January 2017, before finding himself dropped from the side 10 months after that score of 184.
Some 3567 days on from that hundred, Renshaw is now a 30-year-old who looks a far more complete cricketer after earning his third recall as an opener in August.
So good was Renshaw's innings, Australia may have another selection headache after Nic Maddinson came into the side and opened in this Test with a 33-ball duck.
If Renshaw is this settled in the middle order, can they afford to move the Queenslander back to open once Cameron Green or Josh Inglis comes back in?
Australia's selectors had copped heavy criticism on Friday when they brought Nic Maddinson in to open, and he made a 33-ball duck.
But at least the decision to move Renshaw to No.5 paid off, after the Queenslander gave a glimpse of what was possible in the middle in the ODI series.
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 a six.
Another big hook shot bounced onto the hill at Kingsmead, while the left-hander routinely hit through gaps on the legside.
He 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 (5) Lyon and Hazlewood saw out 226 balls between them and lasted a combined 388 minutes alongside Renshaw.
Even after he was out, Renshaw and Hazlewood (5no) added another 26 for the last wicket.
The other concern for the Proteas was the lack of purchase they got out of their spin, after going with two tweakers.
While Harmer (1-68) and Keshav Maharaj (0-75) are there for when the pitch dries later in the game, the fact they did little damage was notable.
That meant Nortje (2-88) was forced to bowl 28 overs in his first first-class game in three-and-a-half years, while Wiaan Mudler (4-68) bowled 27 and Jansen 31 for his 3-68.
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