Renshaw’s career-best score helped Australia recover from 124 for six to make 390 in their first innings and comes almost 10 years after his only other test century against Pakistan.
"I never thought I'd be here this long," the 30-year-old said. "I think once I took a few years out of the team, and had an opportunity in 2023 and didn't take it, you probably think that you're never going to get this opportunity again.
"When I hit the ball (for a century), I was screaming at it, ‘Go, go, go!’ And it just hit me at once. And Nathan (Lyon) at the other end is pretty emotional. So to see that, it was pretty special."
Renshaw had scored single figures in 11 of his previous 13 test innings before this one dating back nine years.
"I always wanted that second one (century). I had the monkey off my back early with that first one but, for me, this one was about all the stuff that has come in the last 10 years. All the hard work, all the failures, all the success."
Renshaw believes the lows in his career provided valuable experience in Durban.
"All the stuff that I've done in the past 10 years has probably led me to this moment. I wouldn't be where I was without all the failures," he said.
"We're quick to judge failures, but I think some of the best learning has come from failures.
"I felt calm at times. I didn't probably look calm but I think everything led up to this and culminated in this innings."
With South Africa 27 for one in reply and trailing by 363, Renshaw expects the wicket to get trickier to bat on.
"It probably got a little bit slower," he said. "There were a few balls where there was a fair bit of spin. A few went low. And I think with the new ball, you saw there was a little bit of movement around."