The world will be watching Saturday’s “dream” Rugby World Cup clash between the top two sides in the world, says South Africa captain Siya Kolisi.

A titanic battle is expected at the Stade de France, where the Springboks’ title defence faces a stern test against an Ireland side coming off a Six Nations Grand Slam.

The result will have big consequences in Pool B, with Ireland yet to face Scotland, who were defeated in their opening match against South Africa.

"This is as big as it gets. We are playing the No. 1 team in the world in the World Cup, that is what you dream about as a child,” Kolisi said.  

“They are an amazing side. They have won 15 in a row so it's going to take the best of us to beat them.

“I am looking forward to it and I can see the excitement all around the world, everybody is keen for this game."

Captain Johnny Sexton, who became Ireland's all-time leading points scorer in their 59-16 thrashing of Tonga last weekend, will be key to his country's hopes. 

“He is a great leader for Ireland. He is also an amazing and important player for them,” Kolisi added.

“You can see when he is there, they are a completely different team. Same for his club. 

"For him to be doing it at his age is remarkable. I would love to still be playing at 38 in a World Cup too.”

One of the major talking points going into the game was South Africa’s decision to pick a controversial 7:1 split of seven forwards and one back on the bench.

The move, which the Springboks have previously used once in a warm-up game against New Zealand, goes against the standard 5:3 or 6:2 ratios, but head coach Jacques Nienaber batted away any criticism.

"I think if there is innovation in any sport it gets a reaction, positive or negative,” he said.

“This is unique, it is the first time a team has named seven forwards and one back on the bench so that is why I would say it's innovation.

“That will get a reaction. In terms of player safety, I don't get that. I know nothing stops anyone else doing it and it will be a sad day if you're innovative in the laws of the game and then they would change that.”