Centuries by Ryan Rickelton and captain Temba Bavuma have anchored World Test Championship finalists South Africa to an imposing 4-316 against Pakistan in the second Test on Friday.

Rickelton batted the entire first day on Friday for an unbeaten 176 off 232 balls, while Bavuma made 106 on a batting-friendly Newlands pitch.

Their 235-run fourth-wicket stand drained the energy from the Pakistan attack, who started well with three wickets in the space of nine runs in the morning but then took only one wicket after lunch when Bavuma gave wicketkeeper Mohammad Rizwan his fourth catch of the innings.

In a bigger blow for Pakistan, in-form batter Saim Ayub was ruled out of the Test after twisting his right ankle in the field in only the seventh over of the game.

He was taken to hospital for scans, and the Pakistan management said in a statement the scans were "sent to specialists in London for further advice on the treatment and time away from competitive cricket."

Rickelton, promoted to opener after Tony de Zorzi was ruled out due to thigh strain, profited on an ideal dry surface against a four-pronged Pakistan seam attack, striking 21 fours and a six.

He and Bavuma put on 112 in the post-lunch session and Rickelton raised his second Test century off 135 balls with his 14th boundary when he whipped Salman Ali Agha to the fine leg boundary just before tea.

Bavuma completed his first Test ton against Pakistan with a single to midwicket, before being dismissed soon afterwards, having hit nine fours and two sixes in his 179-ball knock.

South Africa won the first Test inside four days at Centurion to reach their first WTC final, but had a difficult start on Friday.

Aiden Markram looked shaky for his 17 before giving an inside edge to Khurram Shahzad.

Wiaan Mulder fell to Abbas when the seamer found a thick outside edge, and Salman got Tristan Stubbs caught behind for a duck in his first over.

But Rickelton completed his half-century in the last over before lunch in the company of Bavuma, and the pair dominated the rest of the day.

Kwena Maphaka was among the three changes by the Proteas and the fast bowler became the youngest South African to make his Test debut at 18 years 270 days. The previous youngest was Paul Adams, who was 18 years 340 days when he debuted in 1995 against England.

The home team, who have six successive Test wins, opted to go with the express pace of Maphaka in place of Dane Paterson, who was dropped after picking up his second successive five-wicket haul at Centurion.

Corbin Bosch was also left out after his stunning debut in the first Test when he took a wicket with his first ball and made a scintillating unbeaten, first-innings half-century to give South Africa a match-winning lead of 90.