Russell labelled Friday at the Hungaroring as a “disastrous day” when he came away from FP2 in only eighth and 0.9s off the pace, while team-mate Lewis Hamilton was down in 11th.
The race simulations on the medium tyre were similarly adrift, with Russell’s average lap time over a 10-lap stint some 1.2s slower than the performance shown by Max Verstappen on the same compound.
Russell reckoned: “Our high fuel pace was probably the worst it's ever been yesterday”.
That led the team to “turn the car upside down” and work until 11pm, with the Briton then landing a shock maiden pole by 0.044s over Ferrari’s Carlos Sainz - although a DRS issue restricted Hamilton to seventh.
But Russell does not expect the Mercedes W13 to show an advance on its one-lap pace on Sunday, despite its recent improvement in race trim over the last three races - culminating with Hamilton leading Russell for a 2-3 double podium last weekend at Paul Ricard.
The former Williams driver said: "You take each day at a time; you don't want to get carried away with yourself and you just focus step by step.
“Getting off the line, good start, good first stint - is it going to be a one-, two- or three-stop?
“We know we have, relatively speaking, a faster race car than we do qualifying car. If that's going to be the case again this weekend, I'll be very shocked.
“I think we just absolutely nailed today 100% and got every last millisecond out of it.”
However, team boss Toto Wolff did reckon a win was achievable at the narrow track.
Russell remained coy when asked to elaborate on the overnight overhaul that had been given to the W13 but revealed: “[There were] chats of overall philosophy and if we're going in the right direction as a team. We've been closing the gap.
“But obviously yesterday was a disastrous day.
“We believe that there were many reasons as to why, and they all added up to make us well over a second off the pace. But that felt like we were being a bit generous.
“But then to have a day like today, maybe P1 on pure pace - obviously Max had a bit of an issue [power loss, 10th fastest]
“We definitely turned it around and we're probably back to where we kind of hope to be.”
Russell also added that it would be “meaningless” if he beats Hamilton to the honour of becoming the first driver to score a win for Mercedes in 2022.
