Nel took 4-39 as the visiting side gained a huge 303-run first innings lead and were 43-1 in their second innings at close of the second day's play against Sri Lanka 'A' here on Sunday.
Resuming on 351-3, South Africa declared at 465-4. The home side only managed the wicket of Hashim Amla (150), who was trapped leg-before by seamer Akalanka Ganegama.
Amla batted for over five hours, hitting 16 fours and a six off 272 balls.
Captain Ashwell Prince and A.B. de Villiers too got useful batting practice hitting contrasting unbeaten half-centuries.
Prince, 15 overnight, took 117 balls for his 50 while de Villiers reached 50 off only 46 balls with 10 fours.
After an excellent show by the batsmen, South Africa's seamers struck at regular intervals as Sri Lanka 'A' were bowled out in 46.3 overs. But South Africa decided not to enforce the follow-on.
South Africa 465-4 decl (J. Rudolph 157, H. Amla 150, A. Prince 50 not out, A.B. de Villiers 50 not out) and 43-1; Sri Lanka 'A' 162 (U. Chandana 51 not out, M. Van Dort 29, A. Nel 4-39, M. Ntini 2-34)