Witter's timely three capped a Creighton comeback after Rhode Island had posted a 45-30 lead at halftime.
Witter finished with 10 points, while P'Allen Stinnett and Booker Woodfox each scored 22 for the third-seeded Blue Jays. Creighton will play the winner of Wednesday's Florida-San Diego State contest.
Will Daniels led the sixth-seeded Rams with 16 points.
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