Samson century takes India to 296 in series decider
Samson made 108 in an Indian total of 296 for eight after the tourists were sent in to bat on a slow pitch in hot weather.
The right-handed Samson and left-handed Tilak Varma (52) put on 116 for the fourth wicket.
Samson and Varma came together after India made a brisk start, scoring 101 but losing three wickets inside the first 19 overs.
It took the pair 88 balls to post 50 but they added another 66 off 48 balls before Varma was caught at deep backward square leg off left-arm spinner Keshav Maharaj.
Samson reached his century off 110 balls with six fours and two sixes and added another six before he skied a catch to mid-off.
Rinku Singh made 38 and led some brisk scoring by India's lower order batsmen as the last five overs yielded 52 runs.
Brief scores:
India 296-8 in 50 overs (S. Samson 108, T. Varma 52; B. Hendricks 3-63, N. Burger 2-64)
Match situation: South Africa need 297 runs to win
Toss: South Africa