The victory lifts the Proteas into first place in the tournament after bouncing back from a opening day defeat to hosts Namibia with two comfortable wins against Zimbabwe.
After opting to bat first at the toss, Pretorius and opening partner Jordan Hermann gave the Proteas a quick start with the willow, putting on 59 in the powerplay.
Hermann was the first to fall for a 25-ball 28, the left-handed batter slicing off-spinner Wessly Madhevere (3-20) straight in the hands of backward point.
Dewald Brevis walked to the crease in hot form having a made 75 in both previous encounters and immediately looked at ease as he made his way to 41 from 27 balls.
Brevis and Pretorius put on on stand of 86 for the second-wicket as the Proteas looked well on course to be the first team to breach the 200-run mark in the tournament.
Zimbabwe seamer Brad Evans (2-33) made the breakthrough, removing the dangerous Brevis and the Proteas then suffered a mini-collapse as they lost their next six wickets for 31 runs to finish with a total of 185-7.
Zimbabwe's run-chase got off to a disastrous start as they slipped to 54-7 with Fortuin (4-24) at the forefront of the collapse, while lanky seamer Duan Jansen continued to make a good impression with 3-21.
Captain Sikandar Raza showed some resistance with 45 while some lower-order boundaries from Newman Nyamhuri (27) meant Zimbabwe got up to a total of 142 all out.
The next game in the T20I tri-series will be on Thursday when hosts Namibia take on Zimbabwe in a crucial clash in Windhoek.
Travis Head struck a century as Australia beat Zimbabwe by 84 runs in the second ODI in Harare on Friday, with the visitors taking an unassailable 2-0 lead in the series with a game to spare.
The Proteas have suffered another injury blow in the fast-bowling department after Lungi Ngidi was ruled out of the three-match ODI series against Australia due to a right hamstring strain.
Matt Renshaw and Nathan Ellis starred as Australia opened their three-match ODI series against Zimbabwe with a 59-run victory in Harare.
Ryan Rickelton and Temba Bavuma shone with the bat in the opening round of the domestic first-class competition, but there was an injury concern for Proteas fast bowler Lungi Ngidi who hobbled off injured.
