England pushed through to 425 all out to give themselves a slender lead of just six runs but were thwarted by downpours throughout the afternoon as the Proteas survived to 39 without loss at the close.
But despite having one day left to play in the second Test, Prior, who hit a characteristically attacking fifty, is adamant England can pull off what would be a stunning victory.
“We know that if we come in hard tomorrow and put them under pressure you never know what can happen,” the 30-year-old told Sky Sports.
“If we get a few early wickets we’ve got an opportunity to push hard and win.
“It’s a bit disappointing to not get a bit more of a lead but having said that, to get just ahead was good.”
Overnight centurion Kevin Pietersen failed to add to his 149 as he was trapped lbw by Morne Morkel.
However, Prior was determined not to let Pietersen’s good work on Saturday go to waste and hit a quick-fire 68, before falling to spinner Imran Tahir, to give England some impetus.
Tahir was the tourists’ top wicket-taker in England’s innings, finishing with figures of 3-92.
With first innings’ centurion Alviro Petersen nursing a hamstring injury, former Yorkshire batsman Jacques Rudolph (21 not-out) accompanied skipper Graeme Smith (17 not-out) to the crease for the Proteas’ second innings.
Despite some tight bowling in dank overcast conditions, England’s all-seam attack failed to make any breakthrough in between the rain delays, and it will take something special to see Andrew Strauss’s men force a result on the final day.
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BIOGRAPHY: Lucas Torreira