Much depends on the undefeated pair, Alastair Cook, who registered his first half century of the series, Joe Root and the rest of the order. They do not want to be 100 or more in arrears at the end of the first innings as the pitch will only get more capricious.
The day was set up by De Kock who swashed his buckles to a brilliant undefeated century. Early wickets were crucial for England and they came. Temba Bavuma and Kagiso Rabada went in the first 30 minutes and it looked like England would bowl the hosts out for much lower than 400.
The counter attack came from De Kock who played with gusto and was well supported by a stubborn innings from Kyle Abbott. England had their chances but when a player is launching aggressively there will be moments when a hard catch needs to be taken.
Ben Stokes could not hold on to a hard chance at gully early off De Kock and there were mix-ups between wicket keeper Jonny Bairstow and first slip Cook. De Kock was on 80 at the time but it merely highlighted a growing concern about Bairstow’s glovework. He also contributed to Cook dropping Amla early on the first day so he is under pressure.
None of this should detract from De Kock’s innings though, as he rattled along at a run a ball and pressed England’s bowlers to come up with answers. Width was flayed, runs were scampered and England looked increasing ragged.
In fact they have been rather off the pace all game. Four Tests in as many weeks is hard work and when the series is already won it is only natural that standards will slip a little.
South Africa will not care though. As a team they are in transition and in need of some spark. All sides need regeneration from time to time and that is why this series should not be considered a failure for the hosts, as they have re-discovered De Kock and discovered Rabada. These are excellent talents and will form a good spine of the side for the next few years.
England have already found theirs and during this series they have started to show their worth. It is no surprise that Stokes is central to it. Even when the team is struggling he manages to impose himself, finishing with four wickets for only 86 runs. His importance is such that if he had worn a South African shirt this series they would probably be winning.
He will be needed today and every run he scores will prove vital as England must finish as close to parity as possible. The seamers will benefit as batters start to mistrust the bounce and Dane Piedt has already started to extract some substantial turn. Root and Cook negated it well during the evening but it was such that a flurry of wickets would not be a surprise.
And for all there should be a blanket command: Do not watch Nick Compton’s dismissal. The ball barely bounced 12 inches off Rabada and left him hapless as it thudded into his pads. He had no chance, done by the lack of bounce and it is things like that preying on batters’ minds that precipitate collapses.
So there is much to play for in the next three days. England will not want to tarnish a brilliant series win with a loss; South Africa need a revitalising win and many players are playing for their spots on both sides. Interesting times for all.