AB de Villiers's sparkling half century could not prevent Sri Lanka from snatching a five-wicket victory with one ball to spare as they capitalised on six dropped catches by South Africa in the third Twenty20 on Wednesday.

Niroshan Dickwella belted a career best 68 from 51 balls and an unbeaten 37 off just 16 balls from Seekkuge Prasanna helped Sri Lanka to claim a 2-1 series win as South Africa's victory hopes were dashed by their woeful fielding.

The hosts, resting a number of their leading players, posted 169 for five in 20 overs after winning the toss and electing to bat.

But the fielding blunders from the home side, including three in consecutive balls off a farcical Wayne Parnell over, allowed Sri Lanka to reach their target of 170.

South Africa's innings was anchored by a fine 63 off 44 balls from De Villiers, who warmed to his task after a slow start as he returned from seven months on the sidelines with an elbow injury.

Batting at number three, he put on 71 for the second wicket with Reeza Hendricks (41 from 34 balls) before being caught at point off Nuwan Kulasekara (1-30).

Mangaliso Mosehle (32 not out from 15) provided a much-needed boost to the run rate towards the end of the innings with three massive sixes.

Sri Lanka managed to stay with the rate as South Africa gifted them life after life with a sloppy fielding display.

The home side looked to have the advantage though when the tourists required 52 from the final four overs, but Prasanna continued to hit boundaries in a match-winning display.

They required 11 from the final over bowled by debutant seamer Dane Paterson (0-38 in 3.5 overs), but two boundaries from Asela Gunaratne (11 not out) sealed victory for the elated tourists.

The teams will start a five-match one-day series in Port Elizabeth on Saturday.