The 23-year-old displayed all the attributes that made him one of the standout players in the Guinness PRO14 this season and played a vital role in the match-winning score.
His charge down resulted in substitute Scarlets hooker Ryan Elias diving in for a dramatic late try after South Africa had taken a 20-17 lead with five minutes left on the clock.
Ellis Jenkins, another Blues player who has been influential this season, was also key as he captained Wales for the first time – pinching the ball in the lead up to the opening try for Hallam Amos.
Debutants get Washington run out
South Africa had seven debutants in their side, including Toyota Cheetahs prop Ox Nché, but they were unable to close the game out despite Travis Ismaiel and Makazole Mapimpi – the latter of whom finished as the Guinness PRO14’s second-highest try scorer last season despite leaving Toyota Cheetahs midway through the campaign – both crossing on their Test debuts.
The American experiment struggled to get going in the opening stages, with Elton Jantjies slotting the opening points for the Springboks on 20 minutes after Tom Prydie was penalised.
Wales were then dealt another blow when Scarlets star Steff Evans limped off the field injured, but his replacement Hadleigh Parkes made an immediate impact.
👍🏻 Good luck
The @CheetahsRugby man and @cardiff_blues No 9 make their Test debuts in Washington tonight as 🏴@WelshRugbyUnion face 🇿🇦 @Springboks #WALvRSA #GUINNESSPRO14 pic.twitter.com/E3O56XT5HW
— PRO14 RUGBY (@PRO14Official) June 2, 2018
After Jenkins secured a turnover from the back of a Springboks ruck, Parkes showed good hands to combine with Ross Moriarty and Jenkins again to see Amos cross the whitewash.
The score gave Wales much-needed momentum and they soon had a second.
Williams shines on international stage
After stringing several phases together, Williams broke two tackles five metres from the line to score his debut try, with the extras from Blues team-mate Gareth Anscombe making it 14-3 at the break.
Ismail gave South Africa the perfect response at the start of the second half, cutting out a loose pass from Amos to run in unopposed and close the gap.
An infringement from the Springboks then handed Anscombe a welcome three points, before Wales missed a try-scoring opportunity after Moriarty charged clean through.
They were made to pay for their carelessness moments later when Ospreys’ Owen Watkin was sent to the sin bin after deliberately knocking the ball dead under pressure from Jesse Kriel.
South Africa took advantage of the extra man to grab their second try, with Mapimpi crossing in the far-left corner after a series of dominant scrums. Jantjies kicked the extras to level the scores.
Elias makes it three in three
The men in green almost took the lead with a third try after putting Wales under severe pressure with their forward might as Ismaiel appeared to cross again in the right corner.
It turned out he had already been forced into touch by Cory Hill when he put the ball down, but the Springboks did move ahead with a penalty from Robert Du Preez in the 74th minute.
The stage was then set for Williams to make his final telling contribution.
He charged down Du Preez’s late kick and the ball feel kindly to Elias, who sealed a third consecutive win for Wales over the Springboks.