The left-handed opener elected to bat after winning the toss and powered his way to 132 as the visitors, 1-0 up in the three-match series, compiled 296 for three in front of a meagre crowd.
Smith clubbed nine fours and three sixes before playing on to paceman Kemar Roach late in the day. Jacques Kallis [ Images ] was unbeaten on 45 at the close while AB de Villiers [ Images ] was seven not out.
Earlier, Alviro Petersen hit an aggressive 52 and shared an opening stand of 99 with Smith on a placid pitch that offered little to the bowlers.
After Petersen fell to off-spinner Shane Shillingford, Smith combined with Hashim Amla [ Images ] (44) for a second-wicket partnership of 112.
Smith was dropped twice and dispatched Narsingh Deonarine's off spin for two consecutive sixes to race into the 90s.
Shillingford, playing in his second Test, also dismissed Amla as he finished the day with figures of two for 91.
South Africa (1st innings): G Smith b Roach 132
A Petersen c Roach b Shillingford 52
H Amla c Bravo b Shillingford 44
J Kallis not out 45
AB de Villiers not out 7
Extras: (lb-2, w-2, nb-12) 16
Total: 296 for 3 (86 overs)
Fall of wickets: 1-99, 2-211, 3-283
Bowling: K Roach 14-2-45-1 (nb-10 w-1), R Rampaul 6-0-32-0 (nb-1), S Benn 18-3-64-0, D Bravo 14-2-42-0 (w-1), S Shillingford 31-1-91-2 (nb-1), N Deonarine 3-0-20-0.