World Cup Pool B, Durban - Starts 1230 GMT Monday
On the evidence of last year's one-day series between the teams, South Africa should record a decisive victory.
They won that series by a 4-1 margin with their only defeat coming at Benoni, when Marvan Atapattu made a magnificent unbeaten 123 for Sri Lanka.
But the form has been reversed at the World Cup with Sri Lanka looking a competent outfit in every game, apart from their shock defeat in Kenya.
South Africa, by contrast, have stuttered along and must win to give themselves a realistic chance of progressing in the tournament.
The pressure of being the hosts, reports of dressing room divisions and loss of form by key players, including Jacques Kallis and Allan Donald have contributed to South Africa's difficulties.
Now I suppose it's heroes or zeros
SA coach Eric Simons
But skipper Shaun Pollock is confident they can put all that behind them against the Sri Lankans.
"There are going to be times at the World Cup when it comes down to crunch games, and this is the most important one for us so far.
"If you're going to win the World Cup, you've got to win the crucnh games - that's what it's all about."
The toss could be a decisive factor with previous day-night matches in the tournament won comfortably by the side batting first.
South Africa will not relish the prospect of facing Chaminda Vaas under light, with the left-armer already having taken 16 wickets at a cost of just 7.8 runs each.
So concerned are they that Western Province left-armer Charl Willoughby was summoned to join their net sessions to give the batsmen extra practice.
Sri Lanka held their nerve to beat West Indies
South Africa look certain to recall all-rounder Lance Klusener, who was rested for their last game against Canada, and must also decide whether to include one of their specialist spinners, Nicky Boje or Robin Peterson.
Sri Lanka's sole dilemma appears to be the choice of Pulashti Gunaratne or Prabath Nissanka to share the new ball with Vaas.
"I think we've got a good chance. The support bowling has been good. We have been able to get the job done, "said Sri Lankan skipper Sanath Jayasuriya.
Kingsmead is opposite number Pollock's home ground and he is hoping local knowledge will help see South Africa through.
They are also buoyed by the knowledge that in their last 11 one-day meetings with Sri Lanka, they have come out on top nine times.
South Africa (from): Shaun Pollock (captain), Mark Boucher, Nicky Boje, Boeta Dippenaar, Allan Donald, Herschelle Gibbs, Andrew Hall, Jacques Kallis, Gary Kirsten, Lance Klusener, Charl
Langeveldt, Makhaya Ntini, Robin Peterson, Graeme Smith,
Sri Lanka (from): Sanath Jayasuriya (captain), Marvan Atapattu, Mahela Jayawardene, Kumar Sangakkara, Aravinda De Silva, Russel Arnold, Jehan Mubarak, Avishka Gunawardena, Hashan Tillakaratne, Muttiah Muralitharan, Chaminda Vaas, Dilhara
Fernando, Pulasthi Gunaratne, Prabath Nissanka, Charitha