Miller's power hitting enabled South Africa to score 126 for six in a match reduced to ten overs a side before Sri Lanka reached 107 for six.
Although Sri Lanka made a good start, with Niroshan Dickwella and Dhananjaya de Silva putting on 59 off 31 balls for the first wicket, they could not maintain the run rate and wickets fell rapidly after the left-handed Dickwella was caught on the midwicket boundary off leg-spinner Imran Tahir for 43 off 19 balls.
"It was a dream debut. I am overjoyed," he said.
The left-handed Miller hit three sixes and three fours. He and new captain Farhaan Behardien put on 51 off 23 balls for the fourth wicket.
Behardien made 31 not out off 18 balls.
"We wanted to give David as many balls as possible and it paid off," said Behardien of the decision to move Miller up to number four in the batting order.
He also praised Ngidi.
Asela Gunaratne held two catches for Sri Lanka, one a good diving effort to dismiss Theunis de Bruyn at backward point, bowled two steady overs of medium-pace, conceding only 12 runs, and ran out Mangaliso Mosehle with a direct hit from mid-off.
"I think the bowlers did pretty well," said Sri Lanka captain Angelo Mathews.
"We got off to the good start we needed but after that we didn't have any partnerships."
South Africa included four international newcomers in batsmen Jon-Jon Smuts and Theunis de Bruyn, wicketkeeper Mangaliso Mosehle and Ngidi.
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