The opening batsman, who hit four fifties and a century during the 3-2 ODI series loss against the Proteas, smashed 78 runs off 52 balls in Paarl.
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Skipper Eoin Morgan was also among the runs (42 off 13) as the tourists reached 202-6 from their 20 overs.
In response, South Africa A were restricted to 158-8, thanks in part to the bowling of Reece Topley.
The first T20 international against the South Africans is due to be played in Cape Town on Friday.