The holders face England in Saturday's semi-final - a repeat of the 2019 final - after which lies the possibility of a fourth World Cup final.
Win or lose over the next two weekends, the curtain will be coming down on a golden era of Springbok rugby.
Erasmus, the bold, punchy, outspoken former Bok flanker will remain as South Africa's director of rugby while Nienaber, the ex-physio, will relinquish his role of head coach, pack his bags and fly to Ireland where he will fill the same role at Leinster.
"Who knows, maybe we'll have a World Cup winning coach joining us as well, who may want to share a few secrets with us?" Leinster forwards coach Robyn McBryde told RTE this week.
Nienaber, who turned 51 on Monday, already has a World Cup title under his belt, of course. A second would be a wonderful way to sign off.
But the move to Dublin will be a step into the unknown, the first time he takes charge of a team in his own right, rather than operating alongside Erasmus.
The two men share a long and storied history. They met on military service just before the end of apartheid in the early 1990s.
While Erasmus went on to star for Free State and the Boks, playing in the 1999 World Cup, Nienaber became a physio, working also for Free State.
When Erasmus quit playing and took up coaching he immediately installed Nienaber as his defence guru.
"His passion, knowledge and work ethic around defence was evident even back then," said Erasmus.
"He's very good with people and in terms of communicating what he wants."
'We trust'
Nienaber's communication skills have been used superbly at this World Cup. Where Erasmus can be abrasive and direct, Nienaber is calm and measured, never saying much that the press can pick apart.
The pairing brought Free State two Currie Cup titles before they went on to work together at the Stormers in Cape Town and Munster in Ireland, Nienaber also squeezing in a stint as defence coach for the Boks at the 2011 World Cup.
It was inevitable that Erasmus would pull Nienaber into the Springbok set up ahead of the victorious 2019 World Cup.
In France, they have been street smart, keeping oppositions guessing before and during matches about selection and tactics.
The defence has been outstanding but perhaps most important of all they have the total respect and full attention of the players.
"We trust them because they have earned that," said Handre Pollard on Wednesday.
"They have earned our trust over years, what they have done and how they prepare.
"When they come up with these ideas there are no questions asked. They give us a reason for why we are doing whatever we do and we just back it.
"We are all in. We trust each other, players to coaches and coaches to players."
Nienaber has put off thoughts of Leinster as he prepares the Boks for Saturday's "massive" encounter with the English in Paris, relishing every moment.
"I think it’s going to be a nice challenge," he said after the one-point quarter-final victory over France.
"In knockouts anything can happen. Knockouts are going to be small margins right to the end."
Small margins come down to small details, which is where Nienaber and Erasmus may have a decisive influence.
"The key thing there is the work they put in," said Pollard.
"Nothing they do is just for no reason. It is all thought out, meticulously planned."
