Hashim Amla was again leading the South African charge as the tourists enjoyed a positive start to the third match of the NatWest Series against England at the Kia Oval.
Amla, who made 311 not out on the same ground during the Test series and scored 150 in the second one-day international on Tuesday, was again causing England problems.
The opener, rated as the world's number one ODI batsman, was 41 not out in a 15-over total of 70 for one.
Graeme Smith joined Amla in an opening stand of 50 before falling to James Anderson.
South Africa, as they had in Tuesday's victory, won the toss and opted to bat first.
Amla got the ball rolling for his side by punching James Anderson down the ground for the first four of the day off the third over.
Smith followed by dragging Steven Finn to the mid-wicket boundary and when Amla took 11 off Anderson's third over, the tourists were looking strong.
Amla went aerial for the first time when he lifted Finn into the leg-side for his fourth boundary and Smith took his side to 50 with four and two off Anderson. The Lancashire seamer took England's first wicket in the same over, clipping Smith's bails as the former captain took an ugly stride down the track.
At the end of the 10-over powerplay, South Africa were 55 for one with Dean Elgar joining Amla.
Ravi Bopara slowed down the scoring with his medium-pacers, while James Tredwell, recalled in place of the rested Graeme Swann, started tidily.