The Springboks, in their first match since winning a record fourth World Cup title, outscored Wales five tries -- including a penalty try -- to one.
Jesse Kriel, Makazole Mapimpi, Bongi Mbonambi and Edwill van der Merwe, the player of the match and one of several Springbok debutants, all crossed Wales' line.
Yet an opening 40 minutes featuring three yellow cards -- two for Wales and one for South Africa -- ended with the Springboks just a point ahead at 14-13.
South Africa, however, 'won' the second half 27-0 to set themselves up for two Tests against Ireland, the only side to beat them during last year's World Cup in France.
Both South Africa and Wales were under-strength due to injuries and absences, with players at overseas teams not required to be released for a Test taking place outside World Rugby's international window.
"It was a beneficial game for us because some players were not eligible for selection and a guy like Jasper Wiese is still out (on suspension), while some of the Japanese players haven't played in a while," said Erasmus.
"But the young guys stepped up to the plate and played really well."
He added: "It wasn't a perfect performance. We made a few errors and there were things that didn't click, but there were others that did.
"The important thing is that we achieved our goal and the players who made their debuts showed that they are capable at playing at this level."
Erasmus said Van der Merwe's experience had helped him make the step up to Test rugby, with the Lions flyer scoring a superb solo try five minutes from time.
"Edwill definitely looked very comfortable. He's 28. He maybe slotted in a little bit easier than some of the other guys I think he deserves everything that comes his way now."
Meanwhile, stand-in captain Pieter-Steph du Toit -- whose last match as Springboks skipper ended in a 22-20 defeat by Wales in Washington in 2018 -- said: "It was awesome to play at Twickenham, and with all the (South Africa) fans here, it almost felt like a home game."
'Optimistic' Gatland
This was Wales' seventh successive defeat, a run that includes a Six Nations wooden spoon.
But coach Warren Gatland remained upbeat ahead of a two-Test tour of Australia after a gutsy display featuring four new caps in Ellis Bevan, Eddie James, Jacob Beetham and James Ratti.
"Absolutely I feel more optimistic about the tour ahead," said Gatland.
The veteran coach was especially impressed by Wales captain Dewi Lake, with the hooker scoring the team's lone try on Saturday.
"I thought he was exceptional in terms of the way he played and led the team", said Gatland.
"I thought we did a lot of good things. There are things for us to work on in terms of staying in the moment and making sure we are switched on all the time."
Gatland's second stint as Wales boss is proving much less successful than his first, from 2007-19, where he oversaw four Six Nations titles, including three Grand Slams, as well as two World Cup semi-final appearances.
But the 60-year-old New Zealander insisted winning still mattered now.
"I understand how important results are and I'm quite happy for that pressure to come on me rather than the players," he said.
"We just want players to get better as individuals at this level and make better decisions in this intensity".
