Brits played a few games at flanker in Super Rugby a few years back but he hasn't been in the back row at test level. At 38, Brits also becomes the second-oldest Springbok to play at a World Cup after Victor Matfield.
Brits and Smith are two of 13 changes Erasmus made to the starting lineup after the Springboks lost to New Zealand 23-13 in Yokohama on Saturday. Wing Makazole Mapimpi and centre Lukhanyo Am are the only two survivors from that defeat, which pressed South Africa into likely having to win all of their remaining games if the two-time champions are going to be sure of a quarter-final place.
Erasmus allowed himself one more reflection on the All Blacks before moving on to Namibia.
"They passed more than us, but we ran more than them, and they certainly played a little more conservatively," Erasmus said. "But when they got the opportunity, they were so clinical and pounced, and that is what you have to admire about them. That is where Steve (Hansen) is just a really good coach.
"If we manage to get through the pool stages and face them again, I think it's things that we can manage to overcome."
South Africa can't look further than Namibia because they don't have to look back further than the last Rugby World Cup for evidence that major upsets happen. South Africa lost to Japan four years ago. The threat of the upset was reinforced when Uruguay shocked Fiji on Wednesday.
"There's no denying we'll start as favourites but we'll get a hell of a shock if we think that's enough to win the game," Erasmus said.
Namibia were exciting in glimpses in a 47-22 loss to Italy in their opening game in Japan so Erasmus' tactical decisions regarding Brits hold some weight. The more pressing priority, though, is the need to rest his big men ahead of what the South Africans sees as their toughest test left in the pool, against the Italians next Friday.
No 8 Duane Vermeulen, blindside flanker Pieter-Steph du Toit, hooker Malcolm Marx and prop Frans Malherbe get the night off against Namibia, as do first-choice halves Faf de Klerk and Handre Pollard.
South Africa and Namibia have played twice before. South Africa won 105-13 in Cape Town in 2007 and 87-0 at the World Cup in New Zealand in 2011.
South Africa v Namibia
Toyota Stadium, 9.45 tonight
South Africa: Warrick Gelant, Sbu Nkosi, Lukhanyo Am, Frans Steyn, Makazole Mapimpi, Elton Jantjies, Herschel Jantjies; Schalk Brits (c), Kwagga Smith, Francois Louw, Lood de Jager, RG Snyman, Vincent Koch, Bongi Mbonambi, Tendai Mtawarira. Reserves: Steven Kitshoff, Thomas du Toit, Eben Etzebeth, Siya Kolisi, Franco Mostert, Cobus Reinach, Damian de Allende, Cheslin Kolbe
- AP