The sides meet at Lord's on Sunday and Cardiff on Wednesday with Pietersen determined to inflict a whitewash on the tourists.
Samit Patel took five wickets and Andrew Flintoff hit 78 not out to inspire England to a 126-run win and wrap up the series comfortably.
England are now chasing only the second 5-0 series win in their history after they triumphed over Zimbabwe by the same margin in 2001-02.
Pietersen said: "The 5-0 thing puts a lot more extra pressure on us as it would take us up to second in the world rankings.
"That is the kind of pressure we want - pressure is a privilege and if we win 5-0 it takes us right up to the top. It is something we have talked about and if we deliver we can win 5-0. The dressing room is very happy place.
"We are putting pressure on their batters and playing to our potential. We're hitting their good balls for four and doing a really good job on them.
"I know South Africa really well and they wanted to beat us. They are very proud people and stubborn people."
South African stand-in captain Jacques Kallis said: "The cricket we are playing is not good cricket but England are playing unbelievable cricket. They are one of the best one-day sides we have played against recently."
Flintoff's 77-ball innings and 31 from Patel carried England to 296-7 from 50 overs and South Africa were never up with the rate as they were bowled out for 170 with Patel taking 5-41 to cap a memorable fourth international match.
Patel said: "It has always been a dream to play for England. As an individual you couldn't ask for more. I played against South Africa for the Lions and although it was always going to be tough they knew I could bat."
The South African chase flickered only briefly when Albie Morkel hit Patel for two sixes in an over but having planted him into the stands for a second time the batsmen offered a low return catch that Patel took low down.
But the tourists were out of the game by the 30th over of their innings when Mark Boucher was fifth man out with only 114 on the board.
Flintoff, adding to the 78 he made in the first game at Headingley, and relishing his role at number five put on a free-scoring partnership of 74 for the sixth wicket with Patel that took the game away from the South Africans.
"His recent form with the bat is another triumph for new captain Kevin Pietersen who has promoted the all-rounder up the order and persuaded Steve Harmison, a crucial figure in this series, out of one-day international retirement.