Faf du Plessis could not hide his disappointment after South Africa suffered a shock series whitewash at the hands of Sri Lanka.

The tourists became the first Asian nation to win a Test series in South Africa after wrapping up an eight-wicket win in the second meeting at St George's Park.

Du Plessis had seen his side earn a first-innings lead, but Sri Lanka – led by Kusal Mendis and Oshada Fernando's unbroken 163-run partnership – sealed a remarkable turnaround as a thrilling encounter finished inside three days.

The Proteas captain was left to lament a sub-par display from his team and offered no excuses for the outcome.

"[It's] very disappointing, up there with the most disappointing from my personal point of view," he said.

"We were poor with the bat right through the series. Both teams underperformed with the bat on two pitches that were conducive to scoring runs.

"Two guys scoring big runs like those two [Fernando and Mendis] should have happened.

"Ninety-nine percent of our dismissals were soft, judgment errors and that's only fingers pointing back at ourselves.

"It is mind-blowing that 31 wickets fell in two days; it felt like a superb wicket. 

"The last two Tests, we were not one as a team. We did not take them for granted ... maybe perhaps the guys were mentally a little bit off. But again, that's no excuse."