And then in the final, Sopoaga didn't flinch. He was better than Barrett that night - more certain, more direct and more confident.
Still, for all that Sopoaga emerged, who thought he was in the initial 41-man All Black squad for anything other than the experience it would give him ahead of next year? With Dan Carter and Slade both leaving and Barrett a contender to play sevens, Sopoaga's time will come next year.
Head coach Steve Hansen didn't do much to alleviate that impression when he was asked whether Sopoaga was a contender to play against the Pumas last week. "It's a big ask to expect [Sopoaga] to come in and take over a role like five-eighth. He's got quite a bit of learning to do before we let him loose."
But let him loose they have and clearly they wouldn't be starting him at Ellis Park with a view to readying him for next year. They are starting him because they have seen enough to believe he could be a strong addition to the 2015 World Cup squad. He's a gifted footballer - kicks goals and beats defenders.
Much of the tactical planning for this year has been built on the notion that certainly in the final quarter of big games, the All Blacks need two first-fives on the park. If they have two decision-makers and two kickers, they can more easily change the momentum of the contest.
That doesn't mean Barrett's place is under threat. He's going to the World Cup because he's such a gifted footballer the All Blacks want him on the park at some stage of big games.
Sopoaga's elevation doesn't necessarily mean Slade's place is automatically in doubt either. Prior to Aaron Cruden being ruled out, the selectors were reasonably confident they would be able to fit four first-fives into their 31-man World Cup squad.
The exceptional rise of Sopoaga - if he nails his work at Ellis Park - perhaps keeps that scenario alive and means that maybe both Ryan Crotty and Malakai Fekitoa are in danger of missing out; or that only three specialist outside backs - Ben Smith, Julian Savea and Israel Dagg - will be picked, with Slade, not a specialist but a capable fullback-wing, considered the fourth.
Whatever transpires, Sopoaga is suddenly the man of the moment - the swing player who could have a dramatic impact on the final make up of the World Cup squad.
All Blacks team to play South Africa at Ellis Park on Sunday, kick-off 3.05am NZT, is: Israel Dagg, Ben Smith, Conrad Smith, Ma'a Nonu, Charles Piutau, Lima Sopoaga, Aaron Smith, Kieran Read, Richie McCaw (c), Liam Messam, Brodie Retallick, James Broadhurst, Owen Franks, Dane Coles, Tony Woodcock. Reserves: Codie Taylor, Wyatt Crockett, Ben Franks, Sam Whitelock, Victor Vito, TJ Perenara, Beauden Barrett and Malakai Fekitoa.