AB de Villiers is unsure whether he will take over as South Africa's Test captain following Graeme Smith's unexpected retirement.

Smith made his shock call to step away from international cricket after day three of the decisive third Test against Australia in Cape Town.

As Smith's vice-captain De Villiers is the most obvious choice to take charge of the world's number one ranked side.

The 30-year-old wicketkeeper-batsman also captains the Proteas' one-day international team, but reports on Tuesday suggested he may not want to over-burden himself by taking over as Test skipper.

De Villiers is the world's top-ranked batsman, despite combining those duties with his work behind the stumps, and on Tuesday revealed he is set to talk with coach Russell Domingo after the series against Australia ends on Wednesday.

"I'm not sure. We'll have to wait and see what happens," De Villiers told Cricket Australia's TV channel when asked about whether he would take over the Test captaincy.

"I'll have a few discussions with the coach and team selectors. If it does come my way it'd be a huge honour."

Twenty20 captain Faf du Plessis is another potential replacement, with the right-hander set to skipper the Proteas in the upcoming World Twenty20 in Bangladesh.

De Villiers admitted he had been shocked by Smith's decision.

"I thought he had another year or two in him," he said.

"Each guy's situation is different. Maybe it's a good time for him to move on. He's obviously got his personal reasons.

"I'd rather look back to a superb career and just say congrats over what he has achieved over many years.

"What he has achieved as captain and what he has achieved away from the game and on the field is just amazing and something to look up to."

Domingo too had been surprised by Smith's decision to call time on his Proteas career, that saw him take over as captain as a 22-year-old over a decade ago.

"I had an inkling that it might be around the corner with what has been happening in his life and it is a lot of pressure doing it for 12 years," he said.

"I'm not entirely surprised but I am a bit surprised by the timing of it.

"He's a guy that has always played under a lot of pressure. He's absorbed a lot of pressure and a lot of criticism.

"He's always found the inner strength to ensure he concentrates on his performances and leads the team irrespective of what perceptions of him are.

"His presence alone is going to be sorely missed."

Smith had described his retirement as the toughest decision of his life after ending his international career after almost 350 appearances for his country.

The 33-year-old told team-mates following the third day's play in Cape Town and tributes soon started pouring in for the left-hander, who has been described as "a legend", "a warrior" and "all-round good egg" by some of those in the game.

Smith said he had been mulling retirement over for almost a year.

He said: "This has been the most difficult decision I have ever had to make in my life.

"It's a decision that I have been considering since my ankle surgery in April last year.

"I have a young family to consider, and I felt that retiring at Newlands would be the best way to end it because I have called this place home since I was 18 years old."

Smith will leave South Africa with a major hole to fill, having been a key fixture in their line-up for well over a decade.

This, his last Test, is his 117th, of which he has been captain for 110 - a world record.

He came into the match having scored 9,257 Test runs at an average of 48.72, while he has also played 197 ODIs, averaging almost 38, and 33 T20 internationals.

Smith, who has also taken 26 wickets for South Africa with his off-breaks, continued: "I have always been someone who has left everything out there on the field for my team and for my country.

"I'm extremely honoured and proud to have had the privilege to lead so many wonderful players and to have been a part of building the Proteas culture to what it is today. It is a culture that every player can be, and is, immensely proud of.''