Amla became the fastest batsman to achieve the milestone of 25 One-day International hundred, taking 151 innings, which surpassed India's Virat Kohli (162 matches).
The 34-year-old batsman entered the Champions Trophy in the midst of a purple patch as he hit two hundreds in the latter part of the IPL and continued with his good form in England.
In the preceding three-match series, he managed two half-centuries even as the Proteas lost the rubber.
Amla scored at a decent pace, taking 115 balls for his knock that featured only five fours and two sixes.
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It was a slow start, though, for the Proteas after being asked to bat with Amla and Quinton De Kock (23) putting on just 44 in 12 overs.
The elegant left-hander nicked a full length ball off Pradeep in the slips.
Plessis was dropped on eight by Lasith Malinga off Nuwan Pradeep, setting the platform for the Prorteas' total.
However, the other batsmen did not really fire. Skipper AB de Villiers came a cropper, managing just four runs, and big-hitting David Miller (18) and Chris Morris (20), too, failed to get going.
Left-hander J P Duminy did his bit in propelling the side with his quick-fire 20-ball 38 with five fours and a hit over the ropes.