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Lionel Messi and the 22-year-old Alvarez were a pair of diminutive predators in a partnership that put to rest any hopes Croatia had of reaching back-to-back finals.
Messi may have grabbed the headlines but Alvarez rose to the challenge on football's biggest stage with a huge performance on Wednesday (AEDT).
He earned a penalty, converted by the 35-year-old Messi, before scoring twice in Argentina's 3-0 victory at Doha's Lusail Stadium.
In the process, Alvarez became the youngest player to score twice in a World Cup semi-final since a 17-year-old Pele scored a hat-trick for Brazil in 1958.
"The match from Julian was excellent, not only because he scored two goals, but because he helped our midfielders," Argentina coach Lionel Scaloni said.
"He showcased an excellent game for his age."
A closely fought first half-hour was entirely turned upside down by Alvarez's runs in a devastating five-minute spell.
When it was over, Croatia's players had been left strewn behind him on the turf, trailing by two goals, with their hopes of a second World Cup final in two attempts in ruins.
Until Alvarez changed the game Croatia controlled much of the play, leaving Argentina's attack little to work with.
Alvarez was suddenly alert to creating danger when Croatia midfielder Luka Modric got in a tangle and the ball came loose for Enzo Fernandez to loop a high pass over the defence.
He was quickly clear and bearing down on goalkeeper Dominik Livakovic, who brought the striker down for the penalty which Messi converted.
Minutes later, Alvarez sparked to life when Argentina cleared a Croatia corner, took the ball in his stride just before halfway and ran directly at the defence.
The surging run seemed to surprise Croatia's full-backs as first Josip Juranovic and then Borna Sosa fell backwards while taking wild swipes at the ball as Alvarez lashed his shot into the net from close range.
"It was a nice goal," Alvarez said.
"I don't usually run with the ball a lot but the situation presented itself."
Alvarez ran toward the corner to celebrate and Messi chased him down, grabbing him in a headlock like a playful big brother.
The pair teamed up again in the 69th minute, needing only each other to pass five defenders, mostly because of Messi's intricate dribble to the byline for a pass into Alvarez's path for a simple finish.
Messi joined France's Kylian Mbappe with a tournament-leading five goals, with Alvarez now on four.
He will surely be at Messi's side again on Monday (AEDT) when they return to Lusail Stadium for the final.
