England remained in control at The Oval as Toby Roland-Jones completed his five-wicket haul on Test debut and Keaton Jennings rode his luck at the top of the order on a rain-interrupted third day.
Roland-Jones (five for 57) was unable to finish off South Africa's first innings, from a start-of-play 126 for eight, in time for England to have the option of enforcing the follow-on as Temba Bavuma (52) extended the tourists' recovery in the third Investec Test.
He did add one more success to his four on Friday, however, when Bavuma was last out in a total of 175 to give England a lead of 178.
They then put on a further 74 for one second time round before forecast rain moved in by mid-afternoon and wiped out 59 scheduled overs - Jennings and Tom Westley, one of England's two other debutants here, sharing an unbroken stand of 44 after the early departure of Alastair Cook.
Batting was far from easy under heavy cloud cover, and Jennings especially needed more than the usual share of good fortune to remain unbeaten.
He got off the mark thanks to an overthrow single from his erstwhile compatriots - the first indication his luck might be in, and one confirmed when he edged Vernon Philander for two fours from successive deliveries, off the inside past leg-stump and then bursting through the hands of third slip as he escaped on six.
After lunch, Jennings was given out caught-behind off Kagiso Rabada on 33 but summoned DRS to demonstrate he had not hit the ball and also could not be lbw either because it pitched outside leg.
At the other end, with Cook already gone for the 11th time in his career and second in this match to Morne Morkel when he was bowled off-stump by a very good delivery from round the wicket, Westley played and missed several times too.
England's second-wicket pair began to find the middle more often, though, Westley the more convincing and easy on the eye, to consolidate an already advantageous decision as the hosts bid to go 2-1 up with one more match to play.
Bavuma had begun proceedings by guiding South Africa past the follow-on mark of 153 in a crucial ninth-wicket stand of 47 with Morkel.
Philander was pronounced fit to come in down at number 11, having recovered from a stomach upset which left him on an intravenous drip in hospital the previous evening.
But South Africa did fine without him, having collapsed at one stage on Friday to 61 for seven after Roland-Jones' memorable four-wicket burst.
Bavuma remained defiant then and returned in a highly skilled mode on his way to a three-hour half-century which he eventually brought up by guiding his eighth four to third-man off Roland-Jones from the 111th ball he faced.
He had just one minor scare on 40, when Ben Stokes almost pulled off the most brilliant of catches at gully, flinging himself to his left to intercept a skewed drive off Stuart Broad - with the total on 148.
Stokes could not quite cling on, and in Broad's next over South Africa were safely within 200 runs of England's total after a Bavuma cover-drive for four.
It was not until Morkel faced James Anderson (three for 25) for the first time that England broke the stubborn partnership, the tall left-hander edging to slip off the back foot to bring Philander to the crease at last.
He stayed there long enough for Bavuma to complete his 50 before the top-scorer was caught behind pushing forward to give Roland-Jones his fifth wicket.