Ireland have been the most impressive of the non-Test nations, pulling off a shock win over England and running both India and the West Indies close before losing.
South Africa, with six points from four games, will be assured of a place in the last eight with a victory while Ireland, with two points from four matches, only have a slim chance of qualifying.
The momentum is with South Africa, who bounced back strongly to defeat India by three wickets in a tense finish in Nagpur on Saturday after losing a close match to England.
"Saturday's win is a massive step forward in the self-belief department for us and we can play with a bit more freedom," Graeme Smith, the South African captain, said yesterday.
"That doesn't mean we think we can just walk in and knock Ireland over. Many of the guys in Ireland have played [English] county cricket."
South Africa's batting looked solid against India, though AB de Villiers, the top run scorer in the innings, is a doubt today with a thigh injury.
Andre Botha, the Irish all-rounder, said he and his teammates are aware of the task facing them and know they will need a little fortune to favour them.
"We know South Africa are one of the favourites to win this World Cup. Hopefully, they will relax a bit and give us a chance," he said.
"We know we have to win the last two games and we are not going to back out. Everyone's up for the challenge."
Ireland not only made India battle for victory with their gritty display, but also gave a scare or two to the West Indies before losing by 44 runs in Mohali.
"It's not just the West Indies game, we were in with a chance even against Bangladesh and India, but failed to get over the line," Botha said.
Ireland's experienced paceman Trent Johnston is set to return from injury, after the Australia-born seamer had picked up a knee injury against India and sat out the defeat to West Indies.