South Africa secured a comprehensive 75-run win in the first of the three-match series in Colombo on Sunday, but the tourists slumped to a humbling 87-run defeat in Wednesday's second encounter.
It means the series is nicely poised ahead of Saturday's clash in Hambantota, with South Africa looking to avenge a 4-1 ODI defeat in Sri Lanka 12 months ago.
To do so, more support will have to be given to opening batsman Amla, who has scored centuries in both matches.
However, the 31-year-old was unable to forge a partnership of any real note in Pallekele on Wednesday.
JP Duminy, David Miller and Ryan McLaren all failed with the bat, after Quinton de Kock and Jacques Kallis had too been dismissed cheaply.
Captain De Villiers' knock of 29 proved to be South Africa's second highest total, after they slumped to 180 all out chasing 268 for victory.
"Hashim played an amazing innings and not enough partnerships around him," De Villiers reflected. "I got out to a soft dismissal.
"I got out but there are other batsmen too. It's never nice to win one and then lose one - you look like an inconsistent team. We should have got closer. I don't like us to lose not being in the game the last 10 or so overs."
One worry for South Africa is the form of veteran all-rounder Kallis, who has scored zero and one in the first two matches, and has not yet bowled in the series.
Sri Lanka will have concerns, too. Having lost their final five wickets for just 13 runs in the first match, the second clash saw them go from 256-5 to 267 all out.
Tillakaratne Dilshan was the star in the series-levelling triumph, contributing 86 with the bat and finishing with 3-40 bowling figures.
Victory for the hosts would continue a fine run of form for Sri Lanka in all formats, including defeating England in ODI and Test series' in their recent tour, as well as their fine triumph at the ICC World Twenty20 in April.