Sri Lanka mauled the Proteas batting, the tourists managed just 73 all out in their fourth innings, and Du Plessis conceded that they failed to execute their tactics efficiently.
Du Plessis noted that more than any demons in the surface, South Africa were undone by how Sri Lanka's spinners went about their business.
Left-arm spinner Keshav Maharaj took 4-58 for the touring side.More news: Jabari Parker on the verge of offer sheet from Bulls
Vernon Philander (22no), Aiden Markram (19) and Quinton de Kock (10) were the only South Africans to reach double figures, with Hasim Amla out for a two-ball duck.
South Africa were all out for 73 in their second innings, their lowest since their readmission to global cricket in 1992 following a ban for apartheid, in an abject batting capitulation. South Africa 126 in 54.3 overs (du Plessis 49, Dilruwan four for 46, Lakmal three for 21).
"There were one or two more expansive shots than we would normally play, but the thinking behind the batting was to try and put some pressure back on the bowling, because they don't give you anything". He picked up a wicket in the first innings before dismissing Sandakan in the second innings.
The Galle pitch, though spinner-friendly, was not outlandish and the bowlers relied more on accuracy than mystery. I think I always want to play positively and back my strengths.
While the scorecard reveals South Africa were outspun - losing 17 of their 20 wickets to Sri Lanka's tweakers - it does not say how or why the wickets fell.More news: Phillies Reportedly Increase Offer For Manny Machado
For South Africa, the positivity of the morning session, which had seen Dale Steyn join Shaun Pollock atop the list of the country's top wicket-takers and Kagiso Rabada get to 150 Test wickets, quickly dissipated.
South Africa had begun the day 272 runs in arrears, but had bowled with discipline to remove the last six Sri Lanka wickets for 79 runs. I think often it was the ball that didn't spin as much as we thought that got us out. We expected it to be tough coming here.
Get your defence in order, but when the ball is there to hit, make sure you score.
This was Sri Lanka's day as we saw a good game in that final session from the home team. After a wicketless first innings, Maharaj took four in the second while Rabada's ability on pitches that offer very little to the quicks was impressive.
The second test starts Friday in Colombo.More news: 10,000 protest Trump in Edinburgh, Scotland