Captain Laura Wolvaardt's brisk 60 off 47 balls and Tazmin Brits' unbeaten 55 off 42 hurtled South Africa to 125 without loss in just 14.5 overs on Friday.
It enabled the Proteas to reach their revised target of 121 with 5.1 overs to spare as they rattled up a fourth successive win.
Sri Lanka opted to bat first and were 2-46 after 12 overs when rain stopped play for more than five hours.
Play resumed just before the cut-off time and the match was reduced to 20 overs per side. Sri Lanka added 59 runs in their eight overs to end up at 7-105.
"Very relieved we got a game in the end," Wolvaardt said. "It was frustrating sitting out for four or five hours but we're happy with the two points.
"We just treated it as a normal T20 game and wanted to get ahead of the game as soon as we could and not leave it late."
South Africa rose to second in the eight-team standings, a point behind Australia, who have already qualified for the last-four.
But Sri Lanka have yet to register a win, with just two points from two rain-abandoned games against Australia and New Zealand.
Of the eight matches in Colombo so far, three have been rained out, and Sri Lanka-South Africa was almost the fourth.
Before the rain, Sri Lanka's Vishmi Gunaratne had been struck on the knee while taking a single and was taken off on a stretcher.
At least, the delay gave her time to recover, and she returned to top-scored with 34 before falling on the final ball.
Fast bowler Masabata Klaas showed impressive swing to take both wickets before the rain but couldn't bowl after the resumption because of the revised rules under the DLS method.
In the chase, Wolvaardt and Brits were aggressive against the seamers and used their feet well against the spinners as they smashed 12 boundaries. Brits added two sixes.
The Sri Lanka bowlers found it difficult to grip the wet ball and Brits hit the winning runs by smashing Piumi Wathsala for a six over mid-wicket, which also brought up her half-century after two successive ducks.
"We have two more games, we want to play well and win those and finish as high as possible," Sri Lanka skipper Chamari Athapaththu said.
"The ball was too hard to grip, especially for the spinners, and we are dependent on spin, so we struggled."
On Saturday, winless Pakistan play New Zealand in Colombo.
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