South Africa paceman Dale Steyn completed a four-wicket haul before Sri Lanka fought back with the ball on day two of the first Test in Durban.
The tourists had resumed at 49 for one, but were bowled out for 191 before the Proteas went on to close their second innings at 126 for four, a lead of 170 despite being bowled out cheaply on the first day.
Steyn had removed Lahiru Thirimanne on Wednesday, and continued to impress as he finished the innings with four for 48 to reach 437 Test wickets.
Kusal Perera was Sri Lanka's leading batsman, making 51 off just 63 deliveries before he fell victim to the veteran seamer, leaving the tourists at 152 for eight and looking likely to fall well short of South Africa's first-innings total of 235.
However the last two wickets added 39 to frustrate the hosts, for whom Vernon Philander and Kagiso Rabada picked up two wickets each.
Sri Lanka then saw some early success back in the field as South Africa opener Aiden Markram went for 28 and Vishwa Fernando, who on Wednesday took a career-best four for 62, had Hashim Amla (16) caught at gully.
Temba Bavuma (three) was trapped lbw to Lasith Embuldeniya before the Test debutant caught and bowled Dean Elgar, out for a duck in first innings, on 35.
South Africa captain Faf du Plessis (25 not out) and Quinton de Kock, unbeaten on 15, guided the home side through to stumps.
