The defeat at the hands of South Africa has all but ended Sri Lanka's chances to reach World Cup 2019 semifinals.
South African skipper Faf du Plessis, meanwhile, said after seven games, South African batsmen finally managed to put up a decent show with the bat in the World Cup.
With just six points from seven games, Sri Lanka still have a mathematical chance to be in the semifinals.
JP Duminy, who scored 76 runs in the series, has jumped from 39th to 32nd spot.
South Africa made a daunting 198 for two and Sri Lanka were beaten by 45 runs according to the Duckworth/Lewis/Stern method when they were bowled out for 137
Reeza Hendricks made 66, his second straight half-century, and captain JP Duminy hit 34 not out off 14 deliveries as South Africa finished the innings strongly.
Sri Lankan captain Lasith Malinga said he hoped his side could restrict South Africa to "150 or 160" and then put on a better batting performance than they have managed in losing all five one-day internationals as well as the first two T20 games.
Malinga said that he would quit one-day internationals after the ICC World Cup 2019 in England and Wales this summer.
Hendricks and Van der Dussen put on 116 off 78 balls for South Africa's second wicket.
Sri Lanka, meanwhile, will be eager to break 6-match losing streak in the limited-overs series on this tour.