Before the start of this series, there were genuine concerns about the depth of South African fast bowling. Even before Dale Steyn retired, Kagiso Rabada appeared to be carrying the bulk of the pressure generated in the pursuit of wickets.Lungi Ngidi was laid low by injury, Vernon Philander was economical in India but ineffective. Anrich Nortje had only just arrived in the scene.On day one of the second Test, Dwaine Pretorius, and Anrich Nortje displayed a layer of aggression and purpose later in the day that kept the pressure on England when Rabada and Philander were rested, and in the end, prevented the visitors from rescuing their innings.Let’s look at the key moments that determined the course of the first day’s play. Dwaine Pretorius added a dynamic to the Proteas bowling attack that they didn’t have before – the ability to extract a bit more out of a deteriorating ball.Vernon Philander takes the wicket of Zak Crawley for 4 in the third over. It was more of the same Philander that we saw at Centurion a week ago; miserly, meticulous and deadly. The wicket set the tone for the rest of the innings and allowed SA an early advantage.After hitting Joe Denly on the helmet in the 39th over, Anrich Nortje, who by now was bowling around 148km/h bangs one in short in the 41st over to the other Joe, Root, who gloves it to the keeper. The wicket was all the more important given that Rassie van der Dussen dropped Root at first slip just two balls earlier.Like he did to Ben Stokes in Centurion, in the first Test, Keshav Maharaj produced an arm ball to get a vital breakthrough for the Proteas. In the 48th over he delivered once again, this time with the wicket of Joe Denly for 38th. His delivery clocked 85km/h and snuck through the gap between bat and pad. Denly had looked settled and building towards a bigger score.Ben Stokes loves batting in Cape Town and showed glimpses of how destructive he can be with some lusty blows off Rabada and Maharaj. But it was Nortje again who made the difference, dismissing Stokes for 47 in the 67th over with a slightly slower delivery than Stokes would have expected.Pretorius has proved his value to the team with his ability to get crucial wickets in the dying minutes of a day’s play. In the 75th over, with the old ball, he dismissed Buttler for 29, caught behind. It was a peach of a delivery given the state of the ball. Buttler had looked dangerous as soon as he walked in and had been looking to build a partnership with Ollie Pope. Pretorius’ aggressive celebration said about the South African bowlers’ attitude after tea. It was another turning point in the match.Again Pretorius showed his ability to extract some life out of the old ball. In the 79th over he angled one into the stumps to breach the defence of Sam Curran for 9, to leave England on 231/7