The Junior Springboks came from 14 points down to beat Wales Under-20 52-31 and secure their place in the Junior World Championship semifinals, where they will face England on Monday, 13 July.
Wales drew first blood inside 10 minutes through tries to Caio James and Steffan Emanuel, both from set-piece, with Carwyn Leggatt Jones converting both to put the defending champions on the back foot early.
South Africa responded through a powerful scrum that allowed Hendre Schoeman to cross under the posts, and Ethan Adams finished a sharp individual effort — chipping ahead off a Welsh knock-on and receiving the return pass from Yaqeen Ahmed — to level at 14-all after 17 minutes. Captain Siphosethu Mnebelele then gave the Junior Boks their first lead via a rolling maul try, with Ahmed's conversion clipping the upright.
Wales lost Jack Pritchard to the sinbin on 30 minutes for repeated offside at the maul, and Heinrich Theron punished them with a rolling maul try shortly before halftime to stretch the lead to 12 points.
Risima Khosa extended the advantage three minutes after the break with a fifth try from slick counter-attacking hands, but Wales hit back through Oasin Lewis on 49 minutes and Tom Bowen in the second half to keep the scoreboard ticking.
Replacement Liam van Wyk turned the match decisively, scoring twice from rolling mauls — his second in the 61st minute making it a 19-point game — before Mnebelele added a second of his own in the 67th minute to bring up the half-century. Ahmed converted six of the eight tries in total.
Mnebelele was yellow-carded in the 76th minute for a dangerous clean-out, and Wales took advantage of an uncontested scrum situation with Lewis Edwards scoring a consolation try in the closing stages to make the final margin 52-31.
New Zealand meet France in the other semifinal on the same day.