England won the toss and elected to field after rain delayed the start of the fifth and final one-day international by two hours here on Wednesday.
With the game reduced to 43 overs a side, England captain Kevin Pietersen had no hesitation in asking his opposite number Jacques Kallis to bat on a ground expected to be conducive to swing.
England has named an unchanged side as they go in search of their first ever search of a first ever series whitewash over South Africa.
South Africa replaced all-rounder Vernon Philander with Justin Ontong.
England leads the series 4-0 and need one more win to climb into second place in the world one-day rankings.
"There's a huge incentive for the lads to play for today and we're up for the challenge," said Pietersen.
"To finish the summer as we are doing at the moment is really good."
Kallis, who assumed the captaincy as Graeme Smith is out injured, insisted he would have batted in the hope of putting England under pressure under the floodlights.
"We would have batted anyway and we need to get a score here and put pressure on England," said Kallis, who has been out of form with the bat in the series.
This is the first time England has played a full international at Cardiff's SWALEC Stadium.
England - Ian Bell, Matt Prior, Owais Shah, Kevin Pietersen (capt), Andrew Flintoff, Paul Collingwood, Samit Patel, Luke Wright, Stuart Broad, Steve Harmison, James Anderson
South Africa - Herschelle Gibbs, Hashim Amla, Jacques Kallis (capt), AB de Villiers, J-P Duminy, Mark Boucher, Justin Ontong, Johan Botha, Andre Nel, Morne Morkel, Dale Steyn