South Africa took three Sri Lankan wickets in three overs to finish in a strong position on the third day of the first Test on Friday.
Steyn took four for 48 as South Africa took a first innings lead of 44. They stretched their lead to 170 by reaching 126 for four at the close.
South Africa, overwhelming favourites against a team seemingly in disarray, were bowled out for 235.
Referring to losing series against Sri Lanka last year, Du Plessis said: “They beat us 2-0 in their own conditions which hurt us quite a bit.
Sri Lanka will be led by opening batsman Dimuth Karunaratne, who scored 356 runs at an average of 118.66 in the home series win
Mulder, 20, was added to the 13-man squad which hammered Pakistan in all three Tests in a home series earlier this season.
National selectors said Chandimal was dismissed to help him "regain his form and be ready for future international tours."
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