Whatever happens in the remainder of this Test - and it is beautifully poised - it surely bodes well for England that, for the second game in succession, one of their young batsmen has produced an innings that might reasonably be defined as great.
Whatever holes there may have been in Joe Root's CV - and for a 25-year-old there are very few - one of them was his ability to flourish against good quality attacks on wickets offering a higher than average degree of bounce. After all, he struggled against an outstanding attack in the Ashes series of 2013-14 and was not entirely convincing as an opening batsman in 2013.
But then he was 22. And here, against four fast men who all bowled in excess of 90mph, with the match in the balance and his team in some trouble, he produced a century of the highest class to go a long way to not only answering any reasonable questions about his ability, but to keep his side in the game.
No doubt, had Dale Steyn and perhaps Vernon Philander been available, Root would have been more thoroughly tested. But South Africa actually bowled admirably - at least until they were rattled by Root and Ben Stokes' aggression - and, on a pitch on which nobody else had reached 50 at the time he came to the crease, Root has looked a class above anyone else in the game. And there really are some very fine players involved in this match.
Root's record at No. 4 and No. 5 - a record now set across a not inconsiderable sample size - is deeply impressive. While there are the usual caveats - pitches these days are generally more batsmen friendly and this was only the second of his Test centuries made overseas - he currently averages 68.24 in 43 innings, more than Sachin or Sobers in those positions. There are still bridges to cross - not least, the challenges of Asia and Australia - but it does seem there is something special unfolding in front of us with the blossoming of Root.
It is Root's ability to score from almost any delivery that renders him such a tough proposition for opponents. Bowl back of a length and he will either force you off the back foot from anywhere between mid-off and backward point, or guide you down to third man. Bowl short and he will pull, cut or hook with relish. And so strong and deft are those wrists that every stroke has a vast degree of scope depending on the placement of the fielders.
But bowling full to him allows the opportunity to drive on either side of the wicket and while he may always favour the back foot, the stroke that brought up his century - a gorgeous cover drive off Chris Morris that would have made David Gower proud - was played with his right knee touching the ground, his left leg well forward and his nose over the ball.
He will always be strong off his pads and punish width, too. But perhaps the key to converting this innings into a century - before this game, he had converted only one of his nine most recent Test half-centuries - was his determination to play straighter and leave the ball when appropriate. So while Kagiso Rabada was driven through mid-on and Morris almost straight past the stumps, Root was happy to leave on length and less likely than previously to play across his front pad. As ever in great innings, the shots not played are as important as those that are.
It all leaves bowlers with very little margin for error.
We saw the effects of that here. An attack who had harnessed the conditions expertly in the first 30 overs of the innings - far better than England's vastly more experienced seamers - suddenly found themselves unsure where to bowl. As a result, they lost their discipline and were punished. The 100 partnership occupied only 86 balls. There have been few quicker century partnerships in Test history.
Root gained enormously from the presence of Stokes at the other end. Stokes, aged just 24, already has the rare ability to intimidate bowlers and, after his brutal innings in Cape Town, reacted to the challenge here the only way he knows how: by counterattacking with ferocious power.
There were those - a joyless, sorry bunch who must begrudge sunsets and rainbows their beauty - who dismissed his double-century at Newlands as the plundering of a flat-track bully. But to see him peppered and pummelled by the wonderfully hostile Morne Morkel was to see him tested and tried. The response - impossibly fierce pulls and an ability to hit back of a length deliveries over mid-off - reiterated the impression that England have a gem in Stokes.
Indeed, as the partnership became increasingly reminiscent of the stand made between the same paid at Lord's in May - coming together at 30 for 4, they added 161 in little over 30 overs against New Zealand - the thought occurred: have England had a pair of such exciting young talents - entertainers as well as fine players - since the emergence of Gower and Ian Botham in the late 1970s?
In the grand scheme of things, AB de Villiers' outstanding fielding - such is his pace and athleticism that he came within a whisker of concocting a run-out from a situation where almost everyone else in the world would have accepted a safe single - scarcely warrants a mention. Suffice it to say, he is one of very few cricketers who can stand at mid-off for much of the day and justify the admission price.
For this was as compelling a day of cricket as anyone could wish to see. Hostile fast bowling; audacious batting; breathtaking fielding: we had it all. This series - played by two fine, well-matched teams who manage to play hard but appreciate the skills of their opponents - is quietly developing into something of a classic. And days like this belong not to the supporters of one side or the other, but all those who love and value Test cricket.
There is another lesson here. If the ICC can find the funding to invest in research into pitch preparation - and it may be that technology has a large part to play - it will surely prove money well spent. If more Test surfaces bore the characteristics of this one, notably pace and bounce, it is likely that we would see far more entertaining games. No-one should begrudge Bethuel Buthelezi, in his first Test as head groundsman, if he wins the Man-of-the-Match award.
Nobody is asking for homogenisation of conditions. Variety will always have value. But, over the last few years, we have seen some wretched, lifeless surfaces (Nagpur in 2012, Trent Bridge in 2014 and, sad to say, Lord's on more than one occasion spring to mind) and nothing is more likely to douse the enthusiasm of a new audience for our great game. Forget the nonsense about appealing to 'the purists'; Test cricket is embroiled in a fight for survival. It needs to reach out and entertain beyond the confines of the 'the purists.' Pitches like this will give it a far better chance of winning that fight.