There were early challenges for the batsmen and South Africa were 70 for three before De Kock was joined by Rassie van der Dussen (52) in a fourth-wicket stand of 144.
Later there was turn and bounce for India's spin bowlers but De Kock was in superb form, hitting 12 fours and two sixes before being caught at deep midwicket off Jasprit Bumrah.
Van der Dussen was caught in the deep in the next over and the innings lost momentum despite a hard-hit 39 by David Miller. Prasidh Krishna, one of four changes in the Indian team, took three for 59 while Bumrah and Deepak Chahar took two wickets each.
Rahul said the changes were a combination of "niggles" and wanting to give other players opportunities.
Suryakumar Yadav, Jayant Yadav, Deepak Chahar and Krishna came into the Indian team in place of Venkatesh Iyer, Shardul Thakur, Ravichandran Ashwin and Bhuvneshwar Kumar.
South Africa made only one change, with seam bowling all-rounder Dwaine Pretorius replacing left-arm spinner Tabraiz Shamsi.