Seamer Vernon Philander struck twice as South Africa reduced Sri Lanka to 6-133 at lunch on day two of the first Test at Kingsmead, a deficit of 102 runs on the first innings for the struggling tourists.
The home side took advantage of the bowler-friendly overcast conditions to take five wickets in the morning session for the addition of 84 runs after Sri Lanka had resumed on 1-49.
Kusal Perera (37no) has led the rearguard from the tourists as he took the attack to the home bowlers, but lost partner Dhananjaya de Silva (23) with the last ball before lunch he pulled a Kagiso Rabada delivery to Duanne Olivier in the deep.
Dale Steyn (2-24) struck the opening blow for South Africa on Thursday when he trapped Oshada Fernando (19) lbw, before the wily seam of Philander (2-32) removed Sri Lanka captain Dimuth Karunaratne (30) with the same mode of dismissal.
The visitors had added only four to their overnight score by then, and were reeling when Philander induced an edge from Kusal Mendis (12) to Faf du Plessis at second slip.
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Niroshan Dickwella (8) top-edged Olivier to Steyn at a fine third-man to be the fifth wicket to fall, before Perera and De Silva added 43 for the sixth wicket with some attacking stroke play.
South Africa were bowled out for 235 on day one as Quinton de Kock top-scored with a fine 80. Seamer Vishwa Fernando (4-62) was the pick of the Sri Lankan attack.
