Russell pinched second on the grid in the dying moments of qualifying in Melbourne as he ran 0.236s adrift of pacesetter Max Verstappen, who led the way for Red Bull courtesy of a 1m16.732s lap.
Team-mate Lewis Hamilton, meanwhile, will head the second row in third place after running 0.372s behind.
That result marked Mercedes’ best Saturday of its trying season, as it awaits significant upgrades to improve its W14 car, but also reflected a notable overnight change in fortunes for the team.
While changeable weather disguised the competitive order on Friday, Russell was 0.8s slower than pacesetter Alonso in FP2 as Hamilton ran to 13th. The duo, as per Saudi Arabia, were notably split in their suspension set-up.
Given the change in fortunes, Russell expressed his surprise, saying: “We weren't expecting that, that's for sure.
“A lot of hard work going on back at a factory, here in Melbourne, and wow, what a session for us.
“The car felt alive, the lap of the end was right on the limit and I'm going to be honest, I was little bit disappointed that we didn't get pole position.
“It's one of those things how your expectations change so quickly in this sport, we probably would have been happy with the top-four, top-five yesterday.
“But the car fell awesome and it goes to show we've definitely got potential still to come.”
Russell did concede, however, that Mercedes was not entirely sure where the uptick had come from initially at least, adding: “We’ve got to understand obviously when you have good days and bad days, but we'll take it.”
Hamilton, meanwhile, called the result “totally unexpected” and a “dream” scenario for Mercedes.
The seven-time champion said: “This is totally unexpected. [I’m] really, really proud of the team.
“George did a fantastic job there today. So, for us to be up on the two front rows is honestly a dream for us.
“We're all working as hard as we can to get right back up the front and so, to be this close to the Red Bull is honestly, it's incredible.”
