After belting the third-fastest Shield hundred in history during some early evening fireworks on Tuesday, Harper kept going on Wednesday morning.
The Victorian wicketkeeper was eventually out for 151, the best score of his first-class career, as the visitors were all out for 278 at the Adelaide Oval.
Victoria were reeling at 4-29 before Harper went to work with a masterclass of controlled hitting.
But former Victorian batter Jake Fraser-McGurk was not to be outdone, blasting 101 from 106 balls, his maiden first-class century, to steer South Australia within distance of the tourists' total.
Fraser-McGurk, who only left Victoria at the end of last season, was particularly impressive when you consider the Redbacks were reeling at 4-16.
Cameos from Ben Manenti (51) and Liam Scott (50) helped South Australia reach 252, trailing Victoria by just 26.
Fergus O'Neill continued his breakout season for Victoria, claiming his maiden first-class five-wicket haul by snaring 5-28.
O'Neill was given excellent support by evergreen veteran Peter Siddle, who caused significant issues for South Australia's top-order by taking 3-53.
In response, Victoria's second innings got off to a rocky start as they had to negotiate a tricky period before stumps.
Test contender Marcus Harris failed for the second time in as many days with the opener out to Wes Agar for six.
Victoria finished on 1-12 with Mitchell Perry sent out as a nightwatchman in place of regular No.3 Will Pucovski.
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