Better be a Bok than Irish, says Erasmus of quarter-final scenario

But the man who led South Africa to World Cup victory in Japan four years ago stressed he was happier to be in his team's shoes than those of Ireland.

South Africa ran in seven tries to Tonga's three in a 49-18 victory in Marseille on Sunday to nab a crucial bonus point to leave them atop the pool on 15 points.

The two teams who will advance to the quarter-finals will be decided when the world's number one-ranked team Ireland -- who beat the Boks last week -- take on Scotland, ranked fifth, in Paris on Saturday.

A win for Ireland would see them finish in top spot and South Africa as runners-up.

However, should Scotland beat Ireland by a margin of 21 points or more, with both teams scoring four tries to earn an attacking bonus point, that would mean all three teams would finish pool play on 15 points.

That permutation would see the Scots topping the group on points difference ahead of Ireland, in second spot thanks to their better head-to-head record against the Boks, in third.

"Basically, as I understand it, if Scotland beat Ireland by more than eight points and Ireland don't get a bonus point then Ireland is out," Erasmus said.

"We would be first, Scotland second and Ireland out. If Scotland beat Ireland 8-0, Ireland are out.

"That's how I see it. I was thinking about that a lot last night."

Erasmus added: "The other permutation, which is the interesting one, is when all three of us are on 15 points. Then the team with the best net-o points (points difference) first of all goes through.

"If Scotland's net-o points is the best, then Scotland will go through and we will fall out as Ireland beat us. First it goes to net-o points, then for the second team it is who beat who.

"For that to happen Scotland must score four tries and beat Ireland by more than 20 points and Ireland must get one bonus point somehow. There are three or four very interesting permutations. But if I was Scotland I'd just want to beat them by eight points and not even score a try.

"If I was Ireland I'd definitely make sure they beat them. If Ireland finish first on net-o points then we go through as we beat Scotland."

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Erasmus admitted that he much preferred to be in the Boks' shoes than Ireland's.

"With full humbleness, and there is no arrogance saying this, but I'd rather sit here than be Ireland knowing that we've been number one in the world all the time and Scotland basically just have to beat them by eight points and they are out of the tournament," he said.

"The score could be 22-12 or 22-13 and Ireland are out of the competition if they don't get a bonus point. Of the three teams we are probably the most comfortable sitting here knowing we have got a two-week rest.

"Look, maybe we have got six months rest if the cards don't fall our way!"

Erasmus confirmed that winger Makazole Mapimpi had been ruled out of the remainder of the World Cup after he sustained a "blow-out fracture" of his eye socket against Tonga that will sideline him for up to six weeks.

"It's sad that something like that happened. It was an accident," Erasmus said of the clash of heads Mapimpi had with Tongan scrum-half Augustine Pulu.

English referee Luke Pearce was widely congratulated for not yellow carding Pulu after describing the clash of heads as a rugby incident.

"We will not just miss him as a brilliant rugby player but what he brings to the team," said Erasmus. "It's sad but that's the game of rugby. Those collisions happen."