Victory for Sri Lanka would have moved them level on points with the tournament hosts but they lost wickets at regular intervals, a number of their batsmen bogged down on a slow pitch as they posted 203 all out.
The paltry target provided few problems for Amla (80 not out) and Du Plessis (96no) in a 175-run partnership which carried the Proteas, already eliminated from contention for the knockout stages, to a thumping nine-wicket win with 12.4 overs to spare.
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One of the main talking points away from the action was when a swarm of bees came sweeping across the ground, prompting players and umpires alike to take evasive action, lying face first on the floor for a couple of minutes. The bizarre passage prompted much mirth but came too late to sting Sri Lanka's passive batsmen into action.
Troubled by a slow surface on a glorious day at Chester-le-Street, Sri Lanka were simply unable to keep the scoreboard ticking along. Only 96 runs were registered between the start of the 11th over and the end of the 40th, which tells its own sorry tale. Eight of their top nine reached double figures but no one exceeded 30 while several fell attempting to up the ante after sluggish starts. Amla and Du Plessis showed them the way by regularly rotating the strike amid the odd boundary.
Hashim Amla has attracted some criticism, perhaps some of it unfairly, for his form in the tournament, contributing just one half-century in six previous innings, his 55 against New Zealand taking up 83 balls. But on a tricky pitch on which scoring was extremely difficult for Sri Lanka, Amla got South Africa off the mark with a sumptuous cover drive, which would not have looked out of place when he was at his pomp. An assured unbeaten 80 from 105 balls followed as South Africa romped to victory.
Lasith Malinga proved to be England's stumbling block at Headingley last week and he made the breakthrough in South Africa's innings in the north-east. Quinton De Kock had found the boundary three times in a punchy 16 but could do little when confronted with a toe-seeking, dipping yorker from the slinger. The type of delivery that belongs in a museum, De Kock was unable to jam his bat down in time and was on his way back to the pavilion after his stumps were disturbed.