Sri Lanka reached stumps on 305-5 in Colombo on Thursday, thanks largely to Mahela Jayawardene's 34th Test century.
The tourists had started well by taking three wickets in the opening session, but Jayawardene put on stands of 99 and 131 with Kaushal Silva (44) and captain Angelo Mathews (63) to leave Sri Lanka well positioned ahead of day two.
But Donald - who took 330 wickets in 72 Tests for the Proteas - is confident South Africa can come back into the Test if they show the right attitude in Friday's early stages.
"It's up to us with what we do in that first hour," Donald said. "We have to earn the right in that hour to be in a good position with the ball. We know we can still pull this back.
"We have been in these situations before where we have gotten off the field with the opposition 300 up with four or five wickets down. We have a new ball tomorrow, that first hour will be an investment session.
"We have to be disciplined; we pride ourselves in our discipline, patience and skills. Our attitude in that first hour is going to be crucial."
Seamer Vernon Philander finished without a wicket, but Donald was impressed with the 29-year-old's application.
"The Vernon factor is always going to be there," he added. "He was searching in those few overs and then hit that Vernon Philander length.
"He created chances, went past the outside edge a lot, he is not far away. It would be sad to see him wicket-less but he is a lot happier with his form in this match."