England set South Africa a daunting 466 to win the final test at the Wanderers and save the series on Sunday.
All expectations are that, with South Africa facing a world-record chase, England will clinch victory and confirm a convincing 3-1 series triumph.
England retained its dominance on the third day in Johannesburg by bowling South Africa out for 183 after the struggling home team was 88-6 overnight.
Mark Wood, in his second test back for England after nearly a year out, took 5-46 to help blast the South Africans out and only Quinton de Kock's 76 was memorable for South Africa.
England captain Joe Root led from the front in England's second innings. He made 58 in England's 248 all out — his second 50 of the test — and although the tourists' batting wasn't as impressive as it's been previously in the series, the lead of 465 was almost certainly more than enough.
Root was dashing at stages in the second innings with five fours and a six and England had controlled every day of the deciding test so far, and just about every day of the series since losing the opening match.
The third day at the Wanderers ended with a rare South African high. Root edged well wide of South Africa captain Faf du Plessis at second slip, but he dived full-stretch to hold a fabulous one-handed catch. Root's wicket gave left-arm quick bowler Beuran Hendricks 5-64 on his test debut for South Africa.
That also ended the day's play.
The South Africans were smiling as they came off the field but England's joy is likely to be lasting and confirmed at some point in the remaining two days of the series.
Root's team has been superior ever since rebounding from an opening-test loss, when half the England team was affected by a flu-like virus. England won by 189 runs in the second test in Cape Town and then by an innings in Port Elizabeth to turn the series around.
England could have enforced the follow-on again on Sunday when South Africa fell for 183 and 217 runs short of England's first innings of 400. Root instead decided to bat South Africa out the game, and the series.
And he did much of the work himself. Root hasn't been his brilliant best this series, but he's been solid as a rock.
If England wins 3-1, it'll be its most impressive margin of victory in South Africa in more than 100 years.
South Africa's woes at the Wanderers were exacerbated by the loss of leading seam bowler Vernon Philander, who left the field in the third over of England's second innings with a right hamstring injury.
It's Philander's final test before retiring from international cricket and it was unclear if he'd be able to return to play any more part in the game on his farewell.
That was the second piece of bad news for Philander on the day. It was also announced he had been fined and given a demerit point for a provocative celebration after getting England batsman Jos Buttler out on the second day.
Earlier, England finally ended de Kock's resistance — the only real fight from South Africa in its first innings — just before lunch to give England a hefty 217-run first-innings lead.
De Kock was bowled by Wood for the second wicket of a triple blow by England late in the day's opening session. Ben Stokes came into the attack to break the 79-run stand between de Kock and Dwaine Pretorius in the previous over. Wood bowled de Kock next over and removed last man Dane Paterson two overs later for his five-wicket haul.
Stokes had 2-47 and Chris Woakes 2-38 on his recall to the England team.
Opener Dom Sibley hit 44 to be the second-highest scorer in England's second innings behind Root. And although England lost wickets regularly, with Hendricks the main recipient, England's position was already almost insurmountable with its huge first-innings advantage.
The highest fourth-innings total to win a test at the Wanderers is Australia's 310-8 in 2011. There's only been one fourth-innings total over 400. South Africa's 450-7 against India in 2013 wasn't enough to win then and won't be enough to win now.
England is set to finish it off.