Du Plessis hit an unbeaten 96 and Amla finished 80 not out as South Africa reached their target with 12.4 overs to spare at Chester-Le-Street.

South Africa bowled Sri Lanka out for 203 before chasing down the total with ease.

Victory for Sri Lanka would have taken them level with hosts and pre-tournament favourites England in fourth place but they remain seventh with two matches to play after the loss to South Africa, who were already out of the reckoning to progress.

Sri Lanka's innings seemingly never recovered from the first ball, when Kagiso Rabada had Dimuth Karunaratne caught by du Plessis.

The Asian side did steady the ship after the early setback, reaching 67-1 before the wheels began to come off.

No player made more than 30 as they posted a miserly total, which South Africa took full advantage of.

Lasith Malinga bowling opener Quentin de Kock brought du Plessis and Amla to the crease and they tore into the Sri Lankan attack.

They knocked off the required runs in 37.6 overs to deal a considerable blow to Sri Lanka's semi-final aspirations.

"It feels bittersweet, it doesn't feel like it means too much," said du Plessis.

"The Bangladesh loss was the one which took the wind out of our sails. 

"It sucker-punched us in the stomach and we took the hangover into our next game. 

"But it could have worked the other way for us if we'd got on a roll."