Cape Town - The look of disappointment of captain Faf du Plessis' face when he was dismissed for 36 (123) on day four of the third Test against England at St George's Park told its own story.

The 35-year-old, considered by many to be one of the finest natural leaders South African cricket has produced, is having a nightmarish run that is threatening his future.

Du Plessis and the Proteas have been woeful in Port Elizabeth, and while the likes of Pieter Malan, Zubayr Hamza and Rassie van der Dussen can argue their inexperience at this level, Du Plessis cannot.

This is his 64th Test match and, with Hashim Amla and AB de Villiers no longer, South Africa needed their captain to stand up and score big runs in this series.

Instead, Du Plessis has carded scores of 29, 20, 1, 19, 8 and 36 in the series with the Proteas top order misfiring constantly.

The dejected body language as he plodded off the St George's Park turf was enough to further raise the question of exactly how much time Du Plessis has left as Test skipper.

The 2020 T20 World Cup in Australia in September is potentially his final participation on the international stage, and before then South Africa only play two others Test matches after this England series.

Those will be against the West Indies in the Caribbean in July, but given how things have gone against England, there is no guarantee that Du Plessis will be there.

There is, of course, no obvious replacement as Test captain presently.

When asked if this would be Du Plessis' final Test series after play on day four, coach Mark Boucher confirmed that he had not spoken to the skipper about his future.

"I've got no clue. He hasn't said anything," Boucher said.

"I don't think so."

The coach did acknowledge, though, that Du Plessis had not scored the runs he would have liked.

"For me his state of mind is going to be a lot better if he gets out there and scores runs. We all know that he is under pressure in the media and from a confidence point of view," Boucher said of his captain.

"The positive for me today is that he actually got out there and gave himself a chance to have a look at the conditions and it looked like he had some good rhythm.

"He'll sit back in the changeroom and look at the situation and be disappointed in his performance today and in the Test match, but I'm sure he'll take confidence from being out there in the middle and facing a few balls."

When play resumed on Monday morning, the Proteas will be 102/6 in their second innings, still a mammoth 188 runs behind England.

Unless there is significant rain, they will lose the match and go down 2-1 in the series heading into the Wanderers.