Cai Evans, the son of Wales and British and Irish Lions wing legend Ieuan, kicked 20 points as Wales Under-18s beat South Africa Schools 30-28 in Stellenbosch.
Fly-half Evans kicked six penalties and converted one of two tries scored by number eight Lennon Greggains.
The hosts rallied late after Wales seized the initiative with tries by Mark Snyman and Morne Brandon.
England and France are also in the Under-19 International Series.
Wales face England in their next game on Tuesday, 15 August, with England having beaten France in their opening game.