South Africa chose to bowl after winning the toss and made immediate inroads in the second over when Tshepo Moreki, one of six debutants in the Proteas team, removed Devon Conway's leg before with his first delivery in test cricket.
The more experienced Dane Paterson had the other Black Caps opener, Tom Blundell, edge behind for 20 but that only brought the in-form Ravindra and former skipper Williamson together on a bright, sunny day at Mount Maunganui.
Showing great patience as South Africa tried to suffocate them with accurate seam bowling, the pair took their time to score and were still safely at the crease at the close of play.
"Pretty special, more than the individual landmark, just contributing to a great day for us as a team," said Ravindra, who will resume on 118 not out with Williamson unbeaten on 112.
"They bowled pretty well and the pitch wasn't that free-flowing. But that's the challenge of test cricket, being able to adapt to what's in front of you. To be able to do it with Kane was pretty cool."
Williamson did get frustrated at one point in his innings and Ruan de Swardt (0-24) will count himself unlucky that the master batsman's attempt to slog the seamer over the covers did not end up in the hands of Edward Moore.
Williamson was soon cracking another four to reach the half-century mark for the 34th time in his 97th test, however, and he converted it into another century with his 13th boundary.
Ravindra also got a life off the luckless de Swardt when he was on 80, his top edge flying towards Duanne Olivier at backward square leg only for the fielder to misjudge it running forwards.
The all-rounder, who was recently named ICC Emerging Player of the Year, was playing his first test for two years and made it one to remember when he reached the hundred mark half an hour before stumps with two runs through backward square leg.
The Proteas team named at the toss was unrecognisable from the side that split a series with India over the New Year as Cricket South Africa prioritises the Twenty20 competition (SA20) on which its financial survival depends.
Moreki deserved better than his 1-81 and all four seamers might have welcomed Neil Brand, who was also making his test debut as captain, offering his left-arm finger spin before the final session.
"Fair play to Kane and Ravindra, they played well," said seamer Olivier. "I feel we bowled really well in the first two sessions and then maybe let it go it a little bit.
"Still four days to go and a lot can happen."
The second Test takes place in Hamilton from 13th February.
