Sri Lanka's openers successfully held out for an hour before lunch on Thursday but still face a long vigil if they are to save the match after being set a target of 488 to win the first test against South Africa.
Dimuth Karunaratne (10) and Kaushal Silva (14) held firm against a barrage of fast bowling at St George's Park to reach the first interval of the fourth day on 27 without loss.
Silva did offer one chance off paceman Kagiso Rabada shortly before the end of the session but his edge failed to find the gloves of wicketkeeper Quinton de Kock.
South Africa still have five more sessions to bowl out the visitors and take a lead in the three-match series, while Sri Lanka will be focused primarily on survival rather than attempting to chase down a record-breaking target.
South Africa declared 50 minutes into the fourth day on 6-406 with a massive lead that gives them every chance to win the contest.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
The home side were already 432 runs ahead when they resumed on 5-351 but with captain Faf du Plessis and De Kock approaching half centuries, they carried on batting and accumulated runs easily, even against the new ball.
They shared in a match-high 129-run partnership before the declaration was made after De Kock was trapped leg before wicket by Rangana Herath for 69. Du Plessis remained 67 not out.
South Africa, who won the toss and opted to bat, amassed 286 runs in their first innings and Sri Lanka scored 205 in reply.
