Quinton de Kock top-scored with 43 as South Africa posted 153 for seven in their 20 overs having been put into bat by Pakistan captain Mohammad Hafeez at the Wanderers.
The visitors had reached 60 for two in 9.1 overs when the heavens opened and ended the game with the sub-continent side needing a further 94 to win from 65 balls.
The second and final Twenty20 international will be played in Cape Town on Friday.
Image:South Africa's JP Duminy (second left) is congratulated by teammates after bowling out Pakistan's Nasir Jamshed
Photograph: Siphiwe Sibeko/Reuters
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