"[Williams] was probably kicking himself earlier," Tietjens said.
"Sometimes the offloads in sevens can be the undoing of you, but then they can also win games.
"Something magic like that can win you tournaments."
Perhaps it can also win gold medals. Williams will certainly take time to learn the art of sevens, just like New Zealand's remade squad will need further tournament to gel ahead of the Olympics.
But with the new boys eased into the action through shorter stints, the team showed enough promise and enough character to leave Tietjens confident of what lies on the road ahead.
"What I really enjoyed today was about how they shared the workload - there was always impact when they came off the bench," he said.
"I understand these 15-a-side guys have to make that transition and their lungs are screaming at times but there was a lot of character and a lot of heart, which was good to see.
"To finish like that against a really good South African team was really pleasing."
Before Williams, as is his wont, stole the headlines at the death, the performance of Akira Ioane would have been the most pleasing of the Super Rugby recruits.
As he proved time and again while playing loose forward for the Blues, Ioane's combination of speed and strength makes him a menace when he finds his way to the flanks, scoring New Zealand's opening try against South Africa to end his day with three.
"He's a big man and so fast and to see him scooting out there for tries is amazing,"
Tietjens said. "Once those guys get sevens-fit they're going to be a real threat and that's very pleasing.
"It's not going to happen overnight - I'm not going to say we're going to win this tournament - but I can assure you these guys will do everything in their power to do as well as we can."
The main threat to New Zealand's chances of a hat-trick of titles in Wellington appear to be Fiji, whose fast and physical squad waltzed through pool play with three big wins. They'll take on the United States in the quarter-finals, while England will play Argentina and South Africa will clash with Australia.