The duo posted 86 runs off 62 balls for the first wicket before Stolk was dismissed by Anesu Kimoriwo for a quick-fire 37 off 27 balls, just a handful of runs short of the target.
That did not deter Pretorius, who finished with an unbeaten 53 off 39 as the junior Proteas reached their target in 14 overs.
Earlier, South Africa won the toss and elected to field in partly cloudy conditions.
The decision by captain Juan James was on the money, as Maphaka ran through the Zimbabwe top-order in an opening spell that saw him bag three wickets for seven runs.
He dismissed Brandon Sunguro and Campbell Macmillan for ducks and clean-bowled Panashe Taruvinga for just three runs within the first power play.
Ronak Patel (32) and Ryan Kamwemba (24) were the only significant contributors to the Chevrons’ score. They steadied the ship with a 53-run fifth-wicket partnership that threatened to bring Zimbabwe back into the game.
But the smiles from the Zimbabwean changing room were quickly wiped away by Riley Norton and Tristan Luus. They turned the scoreboard from 69/4 to 69/7 in quick time, conceding only 13 runs in their first four overs as a bowling pair.
Maphaka returned to rattle the tail, claiming the scalps of Newman Nyamhuri (3) and Ryan Simbi for a fourth duck of the innings.
Luus ended with 3/25 and Norton a miserly 1/7 in five overs.