"For us this is as big a game as a semi-final or final," Australia captain Ricky Ponting told reporters.
"Number one and number two sides going head to head at the World Cup. It doesn't get much bigger."
The game has extra spice because whoever wins will carry two points through to the Super Eight stage.
And for the first time since the International Cricket Council one-day rankings were introduced, Australia are no longer the world number one after five consecutive defeats to England and New Zealand this year.
Instead South Africa, who defeated Australia 3-2 in their one-day series last year, are now officially the top side.
It is a position which clearly gave Ponting some wry enjoyment when he gave his pre-match news conference on Thursday.
"Maybe we can go into this series as the underdogs and not the favourites as we always have been," Ponting said. "Maybe some of the pressure is on them now.
"We think about and talk about putting them under pressure. That's what cricket is all about."
For the first time in the tournament, Australia have been able to select from a full squad of 15 after all-rounder Andrew Symonds was passed fit to play on Friday.
In the one-day arena, Symonds is three cricketers in one. He strikes the ball with ferocious intent at number five, joins Ponting and Michael Clarke as a predatory fielding trio in the circle and bowls off-spin or medium pace.
There is little to choose between the respective batting lineups, with South Africa captain Graeme Smith matching Adam Gilchrist and Matthew Hayden for destructive power at the top of the order and Herschelle Gibbs, who hit six sixes in an over against the Netherlands, the equal of Ponting on a good day.
South Africa may, in fact, have the edge in depth with Justin Kemp coming in at number seven followed by Shaun Pollock.
Australia, with Symonds looking likely to share the spin bowling duties with Brad Hogg, have more variety in their bowling than South Africa who appear a little one-paced.
"This game is not going to define the World Cup, but it's an important game in the World Cup," Smith told a news conference on Friday. "Everyone's been looking forward to this game, and it certainly has the potential to be a cracker.
"Both teams have an awesome amount of talent and respect for each other."
After disappointing attendances at Warner Park, Saturday's match is expected to be a sellout with 10,500 spectators crammed into the stadium in the centre of Basseterre.
"This is our grand finale," St Kitts and Nevis Sports Minister Ricky Skerritt told reporters. Teams:
Australia: Ricky Ponting (capt), Adam Gilchrist (wk), Nathan Bracken, Michael Clarke, Brad Haddin, Matthew Hayden, Brad Hodge, Brad Hogg, Michael Hussey, Mitchell Johnson, Brett Lee, Glenn McGrath, Andrew Symonds, Shaun Tait and Shane Watson.
South Africa: Graeme Smith (capt), Jacques Kallis, Loots Bosman, Mark Boucher (wk), AB de Villiers, Herschelle Gibbs, Andrew Hall, Justin Kemp, Charl Langeveldt, Andre Nel, Makhaya Ntini, Robin Peterson, Shaun Pollock, Ashwell Prince and Roger Telemachus.