The 20-year-old left-hander made the most of a dropped catch at slip by Mohammad Hafeez off Sohail Tanvir when on only two with a 135-ball 112 to bolster South African innings who won the toss and opted to bat on a flat Sheikh Zayed Stadium pitch.
De Kock lifted lanky paceman Mohammad Irfan over long-on for a big six to reach the three-figure mark off 127 balls, anchoring the innings with a 87-run opening stand with Hashim Amla (46) and another 69 for the third wicket with skipper AB de Villiers who made 40-ball 30.
Pakistan, seeking to level the five-match series after going 2-1 down, also missed a golden opportunity to run out Amla on 13, when skipper Misbah-ul Haq failed to break th stumps with the batman well short of his crease on a single.
Amla was finally dismissed by Hafeez, bowled in an attempt to run the ball to third man in the 19th over. Amla hit six boundaries off 50 balls.
De Kock continued in the same vein, reaching his maiden fifty off 68 balls.