Cook leads solid England after de Kock's maiden Test hundred
England 138 for 2 (Cook 67*, Root 31*) trail South Africa 475 (De Kock 129*, Cook 115, Amla 109, Stokes 4-86) by 337 runs
Scorecard and ball-by-ball details
Quinton de Kock announced his arrival among the ranks of attacking wicketkeeper-batsmen to be reckoned with as he raced to an enterprising maiden Test century which has positioned South Africa for a consolation victory on a Centurion surface already showing signs of unreliability.
Only two England batsmen have been prised out in the first 46 overs in their reply to South Africa's 475 and their opponents have looked a pace bowler light, especially with Morne Morkel in one of his malfunctioning moods. But Nick Compton had reason for grievance after he was lbw to one from Kagiso Rabada from scuttled through low - this on only the second day. There was enough evidence that he might not be the last.
England's top three has offered low returns throughout the series: one half-century apiece for Alastair Cook, Alex Hales and Compton. But Cook remained at the close on 67, England's deficit still 337 runs with eight wickets intact, and there were signs as he tried to plot a long route towards safety that that his long-limbed defence was creeping back into rhythm.
Cook has rarely looked more exasperated as in England captain in the field than he has during the 132 overs that South Africa batted. England, with the series won, have lost a little intensity. His batting is all about such qualities, whatever the position of the series, and he will be hell bent upon coming up with a meaningful retort. Apart from an inside edge off Morkel, on 55, which popped safely into the off side and numerous attempts to have him caught down the leg side (the latest theory), he played soundly.
England's reply started badly: Alex Hales perishing in the nine overs up to tea. Hales is fast becoming the latest failed claimant in the past three-and-a-half years to the opening position vacated by Andrew Strauss. Unlike many other contenders, he has a limited-overs pedigree behind him, enough to encourage loyalty by many good judges, but in Test cricket his approach remains fraught with uncertainty, summed up by a weak drive at a floaty outswinger from Rabada and a catch for Dane Piedt at cover point.
Hales, unbalanced in the shot and dismissed for 15, has one innings remaining to enhance a debut series that stands at 135 runs at 19.28 with only one half-century on a Cape Town featherbed.
De Kock now has that settling maiden hundred under his belt. His return to the South Africa wicketkeeping role after a freakish accident that ruled him out of the third Test - he slipped while walking the dogs - had not been universally supported after a strong showing behind the stumps by Dane Vilas as his emergency replacement.
But that debate was silenced as de Kock's blithe-hearted strokeplay took full toll of a lethargic England attack that failed to respond to the rigours of a fourth Test in as many weeks. Unlike his fateful dog walk, this time he had provided a strong lead.
He remained unbeaten on 129 from 128 balls when South Africa's innings finally came to a halt. Such a breakthrough innings could not be more timely for South Africa as they seek to emerge from an uncertain period - illustrated by five changes for this Test - and establish a new-look side.
De Kock survived two challenging catches on 28 when Ben Stokes could not hold on at gully off James Anderson and again on 90, Cook this time the culprit as de Kock drove the offspinner Moeen Ali to short cover.
He was only 62 when South Africa lost their eighth wicket, but Piedt provided steadfast support for more than two hours as the home side, trailing 2-0 in the series, took a firm grip on the final Test.
Just to add to England's disenchantment, there was a mix-up between Jonny Bairstow, a wicketkeeper under scrutiny, and his captain, Cook, who was stood unsettlingly alongside him at first slip when an edge from de Kock, on 80, flew between them. Both could have gone for it; neither did.
The poor alignment of keeper and slip should definitely be questioned before any other factor because Cook had virtually moved alongside Bairstow so confusing their areas of responsibility. On this occasion, it was a tactical error primarily, but it was not the first chance to go astray and the dissatisfied expressions on both faces told of a distrust and disappointment that is unlikely to aid Bairstow's cause.
England needed quick wickets at start of play with South Africa already 329 runs to the good. They missed out on de Kock, who had added only three to his overnight score when Stokes failed to hold on, but had a promising start nevertheless when Stuart Broad and Anderson struck in the first four overs, Temba Bavuma edging to the wicketkeeper and Rabada falling lbw first ball.
Finally, nearing the end of an unrewarding tour, Anderson had made the ball swing and must even have found joy in Rabada's decision to review the decision as it meant he could wallow in innumerable replays proving as much on the big screen. It was the only joy he found in the innings. Other have been equally unimpressive: Chris Woakes has looked trouibled; Moeen Ali's length was all over the place.
De Kock's response to the loss of early wickets was emphatic. A modicum of width was enough for him to flay his next two balls, from Broad, for successive off-side boundaries. A wristy late cut against Moeen signalled his half-century. When Kyle Abbott deposited his first ball from Moeen for a long-on six, and de Kock also cleared the ropes later in the over, South Africa reached drinks in exuberant mood.
Stokes silenced Abbott with a blockhole leg-before - the batsman's review entirely pointless - but de Kock sallied forth. There was no dallying in the 90s as he again found Moeen to his liking , a pull and drive over the top taking him to 99, before stealing a single into the covers to reach his hundred at the start of Moeen's following over.
It was Stokes who eventually moved things on, breaking Piedt's resistance with a rising delivery and Morkel lbw without scoring for his third wicket of the day and figures of 4 for 86.
David Hopps is a general editor at ESPNcricinfo @davidkhopps