It continues a remarkable strike rate that has seen the 31-year-old plunder 12 tries from his last 10 internationals, including two against the world champions.
For Williams - a player dismissed as two small for Test rugby by former Wales and current New Zealand coach Graham Henry - there is a simple explanation for his enduring brilliance.
"I'm enjoying life at the moment and my rugby is going really well because of that. I don't take rugby too seriously," he said.
"I love the game more than anything but my family comes first. I think that is the difference. In the past it was rugby, rugby, rugby.
"I train with a smile on my face and I enjoy my training. I stay healthy and fit and I want to get my hands on the ball."
South Africa made an ominous start that evoked memories of last weekend's 43-17 drubbing in Bloemfontein when Jean de Villiers and Ricky Januarie crossed in the space of six first-half minutes.
But scrum-half Gareth Cooper showed his own finishing instincts to stem the early onslaught and when Williams crossed the match was blown wide open.
"I enjoyed that score as equally as any other I have produced this season," said Williams, who is Wales' top try-scorer with 43.
"It was a crucial time to score it because the Springboks had run in a couple of tries earlier in the first half.
"It just goes to show the effort the lads put in. I enjoyed the try but unfortunately it did not help us to win the game."
"We felt after the 6 Nations success we could have beaten South Africa yesterday but it was not to be," said Williams.
"It's encouraging to come over here and to ambush them and push them as close as we did. The scoreline flattered them slightly I felt.
"It was a very open game and both teams defended very well - it must have been very good to watch.
"It's never easy playing up at this altitude but the conditions were great and suited an open game. That's what we got.
"We were 20 minutes from beating them. With 20 minutes to go we felt we were playing the better rugby.
"Perhaps we lacked a little bit of experience and did not know how to kill games off.
"Not a lot of teams come here and win and we have to take a lot of credit out of that.
"It was not a perfect performance from either side but at least we have gained a little bit more respect from this game.
"We can now have a month off and then a good couple of months training to prepare for another big game.
"If you are going to beat them then you need to perform for 80 minutes."