Johannesburg - Joe Root made a patient half-century as England set South Africa a world record target of 466 to win the fourth and final Test at the Wanderers Stadium on Sunday.
Fast bowler Mark Wood took five for 46 as South Africa were bowled out for 183 in their first innings.
De Kock scored his fourth half-century of what has been a poor series for the South African batsmen.
"If you look at the amount of time left in the game we've still got to look at going for a win," he said.
Vernon Philander was out to the sixth ball of the day, reducing South Africa to 93 for seven, at which stage a follow-on seemed a strong possibility.
He left the field immediately and was sent for a scan.
Despite the loss of their leading bowler, South Africa bowled and fielded better than in the first innings when England scored 400.
Typically at the Wanderers, there was some uneven bounce as cracks opened up, which will make South Africa's task virtually impossible.