In the wave of change that has broken over South African cricket in the last month, one thing has stayed the same: the captaincy. But that does not mean Faf du Plessis' leadership has escaped scrutiny.

The skipper, who last scored a Test century more than a year ago and has not crossed 30 in his last seven innings, will be in the spotlight at St George's Park where either South Africa or England could ensure they cannot lose the four-match series, currently locked at 1-1. The other will go to Johannesburg playing catch-up. If it's South Africa in that position, expect questions over du Plessis' future to multiply, much like they did with his counterpart Joe Root in New Zealand.

It's not entirely unexpected that South Africa have to start their succession planning. Du Plessis is 35, has played international cricket for nine years and has identified this year's T20 World Cup as a potential swansong. While he has repeatedly called himself "driven" to continue as captain in all three formats, he has also indicated South Africa should look at other options, especially in ODIs, so the transition is smooth.

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The only Tests South Africa will play between the end of this home season and the start of next summer is a winter tour to West Indies. Given the schedule and the circumstances, its not unthinkable that this England series could be du Plessis' last at home, or perhaps his last, full stop. And ultimately the decision may not be his to make.

Form will have to come into the conversation at some stage. It already has by those who took offence to du Plessis saying Temba Bavuma had to force his way back into the Test XI by weight of runs when du Plessis' own load has been fairly light. Du Plessis' last seven innings have brought 79 runs with a top score of 29; Bavuma's last seven have brought 102 runs with a top score of 38.

But go back, even just to the beginning of 2019 and compare the numbers. In the first nine innings of the year, du Plessis scored a hundred and four fifties, Bavuma only had one half-century. Despite tailing off at the end of the year, du Plessis averaged 41.41 in 2019 (by way of comparison Bavuma averaged 19.84) which is nothing to scoff at, especially when considering everything else the captain had to deal with.

In the build-up to this series, du Plessis spoke about the "burden," of being the buffer between the players and the crumbling administration. He did not provide concrete examples of exactly what he had to do during the crisis but said he took on more responsibility than usual. After the coaching overhaul, he indicated he was looking forward to simply leading on the field, which is when he believes he is at his best.

Though du Plessis has often claimed captaincy has improved him as a player, the numbers don't emphatically agree. As captain, he averages 40.32 compared to 41.02 when being captained. Instead, we have to turn elsewhere for evidence of du Plessis' ability to steer a ship, like the Mzansi Super League (MSL).

While eyes were initially on Bavuma's Jozi Stars and Quinton de Kock's Cape Town Blitz, as both those men have been mentioned as future national captains, it was du Plessis' Paarl Rocks who won the title. Despite losing two high-profile players, Aiden Markram and JP Duminy, to injury before the competition began and playing with a squad that had almost no recognisable names (had anyone outside of South Africa heard of Kerwin Mungroo or Ferisco Adams?), the Rocks were the best-organised and strategised team in the tournament, a testament to the coaching of Adrian Birrell and the captaincy of du Plessis.

The same clear thinking has not always been on display in the Test series. While du Plessis followed his gut feel in keeping Keshav Maharaj on for an extended spell in the second innings at SuperSport Park - where Maharaj got the wicket of Ben Stokes - the captain did not use his spinner as effectively at Newlands.

Maharaj often bowled to poorly set fields and leaked runs, causing Mark Boucher to call him a "work in progress", which seems a strange thing to say about the second-most successful spinner since readmission. Similarly, Anrich Nortje, known for his bounce, operated without a short leg, Dwaine Pretorius was underused and du Plessis delayed in taking the second new ball in England's second innings, when South Africa needed quick wickets.

Those tactical errors have nothing on the shot du Plessis played on the final day, when South Africa needed him to be at his most defensive in an attempt to save the game. Minutes before lunch, with a second new ball due in under four overs, du Plessis swept hard at a Dom Bess delivery and sent it straight to square leg.

Du Plessis later said he made a "mental error," and that he understood South Africa's senior batsmen need to step up to support the younger players in the top six. The reality is that nobody needs to step up more than du Plessis and history suggests he knows how to do it.

In 2015, the year South Africa hosted West Indies, toured Bangladesh in the monsoon and were heavily defeated in India, du Plessis averaged 16.75. Du Plessis was dropped early in 2016 for the final fixture of a four-match series against England but returned as captain seven months later and scored a century to win a series against New Zealand.

Since then, leadership rather than runs, has been the standout feature of du Plessis' career. In 2018, he averaged just 24.36 but led South Africa to two crucial series wins, over the No.1 ranked Indian side and to a first victory at home over Australia since readmission.

That South Africa lost a rubber in Sri Lanka four months later and a home series to Sri Lanka seven months after that was incidental. With the 2019 World Cup looming, attention was elsewhere. Besides, du Plessis had already saved face with a century in the New Year's Test against Pakistan to take some of the heat off himself, something he has been good at throughout his career.

That's the way he started, with a century in the block-a-thon to draw a game that was all but gone in Adelaide in 2012. That's the way he embraced his promotion to the captaincy with a century to seal his first series in charge, against New Zealand at Centurion in 2016. That's way he responded two months later, with a century in South Africa's first pink-ball Test with the series won and the aftertaste of 'Mint-Gate' still fresh in the mouth. And that's the way he signed off from the World Cup, with a century in South Africa's last hurrah, victory over the much-fancied Australians.

Du Plessis has scored big runs when it matters and the situation he is in now, with South Africa craving a fresh start and his career on the brink of the end, it matters most of all.