South Africa reclaimed the top Test spot earlier this August after it beat the then number-one ranked England 2-0.
For South Africa to retain the numero uno spot, they just need to draw the series, while Australia can return to the top of the tree for the first time in three years if they win the series irrespective of the margin.
The Proteas currently head the table with 120 ratings points, with England in second spot on 117 and Australia at number-three position on 116 ratings points.
The final position for the losing side in the Australia-South Africa series would be determined by the result in the India-England series that starts from November 15.