Heavy rain had delayed the match by two and a half hours before South Africa, sent in to bat, made 78-4 with a top score of 30 by skipper AB de Villiers, while Hashim Amla made 16.
Sri Lanka managed 46-5 in their seven overs.
Both teams have already qualified for Super Eights round after beating Zimbabwe in their opening matches.
South Africa made 78-4 against Sri Lanka in a group C match at Hambantota on Saturday after rain curtailed the affair to a seven-over per side contest.
Skipper AB de Villiers smashed two sixes and a boundary during his 13-ball 30 while Hamshim Amla who scored 16 and Faf du Plessis 13 were the other main run getters after South Africa were sent in to bat.
Opener Richard Levi made four and JP Duminy managed 12 not out.
Sri Lanka were without their ace spinner Ajantha Mendis, who took 6-8 in their win over Zimbabwe, after suffering from a side strain.
Heavy rain delayed the start by two and a hours as a sell-out 35 000 crowd waited anxiously for action, which finally started at 6.00pm (1230 GMT) local time.
Irrespective of the result of this match, both South Africa and Sri Lanka have qualified for the Super Eight rounds after beating Zimbabwe in their opening matches.
South Africa 78-4 in seven overs (AB de Villiers 30); Sri Lanka 46-5 in seven overs (D. Steyn 2-10) – Sapa.