The 20-year-old spinner first starred with ball picking up 2-13 in four overs as Pakistan reached a formidable total of 180-9 on the back of captain Fatima Sana's brilliant 90 off 41 balls.
The match came down to the final delivery with the Proteas needing to two runs to win and Reyneke deposited a six over extra-cover to give the hosts a 1-0 lead in the three-match series.
After winning the toss and opting to bat, the visitors were soon deep trouble at 64-6 in the 10th over.
Reyneke made the breakthrough with just her third ball in international cricket, bowling opener Gull Feroza for 13.
The Western Province allrounder then got rid of the dangerous Natalia Pervaiz, who struck 20 off 17 deliveries.
Pakistan were capitulating but Sana had other ideas and smashed 9 fours and seven sixes in her incredible knock as the tourists reached the 180-run mark.
Home skipper Laura Wolvaardt once again led from the front with the bat as the number 3 struck 61 off 38 ball in the mammoth run-chase.
The Proteas Women then lost a cluster of wickets in the middle overs with Reyneke walking to the crease with score at 145-5 after 16 overs.
After a nervy start, that included three dot balls, the right-handed showed her power to hit three sixes and be unbeaten on 29 off just 16 to take her side over the line.
Pakistan avenged their shock T20 World Cup defeat to the United States from two years ago with a commanding 32-run Group A victory in Colombo on Tuesday, powered by Sahibzada Farhan's half-century and a dominant spin performance.
The Pakistan government has given it's approval for their national team to take the field and play India in their T20 World Cup encounter, scheduled for Colombo on 15 February.
Pakistan survived a dramatic scare to defeat the Netherlands by three wickets with three balls remaining in their opening Group A match of the 2026 ICC Men's T20 World Cup at the Sinhalese Sports Club Ground in Colombo on Saturday.
The ICC has warned the Pakistan Cricket Board of "long-term implications for cricket in its own country" if the national team boycott's the game against India at the T20 World Cup.
