South Africa have played all their group games and currently reside at the summit on 15 points following their victory over Tonga on Sunday.
That 49-18 triumph over 'Ikale Tahi in the Rugby World Cup took them above Ireland in the standings, with the Scots a place further back.
It will likely be a straight shootout between those two Six Nations outfits for who qualifies alongside South Africa, but there is a scenario where the Boks miss out.
Should Scotland beat Andy Farrell's men with a try bonus point and with enough of a gap to overturn the Springboks' points difference, that would take them through as group winners.
But if the Irishmen also take home a try bonus point, that would also put them on 15 points and they would finish above Nienaber's charges due to their 13-8 victory in Paris.
"I think between ourselves, Ireland and Scotland, everybody can still miss out on the quarter-finals, every single team, all three of those teams, but all three can also still make it so the pool will only be decided on Saturday," the head coach said.
"Could I believe in a scenario that they will decide 'do we want to get this amount of points and then get South Africa out of the way?' That would probably be match-fixing, I would say. I hope not.
"Rugby is clean. We wear those t-shirts so hopefully not because that would be extremely disappointing."
Should the Springboks get through, they will have a two-week break until their quarter-final clash.
That could be a benefit, giving them extra time to recover, or put them at a disadvantage, with their last-eight opponents more match ready, but Nienaber sees similarities to their title-winning campaign of four years ago.
"The positive is it is pretty much similar to what we experienced in 2019 when we also finished our pool quite early," he said.
"I think we had a 12-13 day preparation break before we went into our quarter-final against Japan. It is something we have done before as a group. It worked out well for us back then so we'll give them two-three days off and then start preparing.
"Until we know what the outcome is of the pool and who we will face, the France New Zealand pool will only be determined on Friday night so we will only actually know, depending on how [Pool B] finish, we will have a good idea of who we will play.
"I think in the first week now, give the guys some time off to get away from the game, to have a little bit of a mental break and then start focusing on what we can do better. So internally focusing on us on things we must improve if we want to stay in this competition until the end."