In the second T20I in Potchefstroom on Saturday, the Proteas Women were defeated by Sri Lanka by seven wickets.
The visitors achieved their target of 139 with seven balls to spare, thanks to an unbeaten partnership between Vishmi Gunaratne (65* off 57 balls) and Kaveesha Dilhari (45* off 28), both of whom are young players.
Winning the toss, the Proteas elected to bat first and openers Anneke Bosch and Tazmin Brits made a positive start, reaching 26-0 after three overs but Brits, in her 50th T20I, dragged one on to her stumps off Achini Kulasuriya.
Bosch and experienced all-rounder Marizanne Kapp took South Africa to 48-1 at the end of the powerplay. Shortly after achieving her fourth T20I fifty, Bosch fell to Dilhari.
Sri Lanka took three more wickets quickly. Suné Luus was run out and Kapp was caught in the inner ring for 44 by Inoshi Priyadharshani.
Chloé Tryon, Annerie Dercksen, and debutant Karabo Meso were also dismissed as South Africa struggled to 118-7 with three overs to spare.
Stand-in captain Nadine de Klerk scored 16 runs off 16 balls, while Nonkululeko Mlaba scored four runs, taking the team's total to 137-8 after 20 overs. Mlaba was the last wicket to fall, run out once again.
The powerplay ended with Sri Lanka at 37-1, with Athapaththu (six) dismissed by Proteas seamer Tumi Sekhukhune (1-27).
Sri Lanka were restricted to 16-0 after four overs, but Gunaratne led the charge, forming a 31-run partnership with Harshitha Samarawickrama (12) to guide Sri Lanka past the 50-run mark after nine overs.
As the partnership gained momentum, Samarawickrama was bowled by Tryon (1-14). De Klerk then dismissed Hasini Perera (1).
Gunaratne and Dilhari then showed their temperament in a match-winning 76-run stand for the fourth wicket, leaving 70 runs needed off 48 balls for victory.
The series decider is scheduled for Wednesday at Buffalo Park in East London. The match starts at 18:00.