The South African struck 24 boundaries, sharing century partnerships with Luke Wright (60) and Ben Brown (67), as Sussex moved their score on from 74-1.
Jofra Archer then smashed four sixes and three fours to score 42 off 14 balls as Sussex declared on 443-6.
Leicestershire openers Paul Horton and Arun Harinath reached stumps unbeaten, leaving 389 runs to score on day four.
Sussex's Stiaan van Zyl scored an unbeaten 166 as Leicestershire ended day three 36-0 chasing 425 to win.