Mercedes enjoyed a successful Friday at Suzuka as Russell led a 1-2 finish ahead of teammate Lewis Hamilton in a wet second practice, although he warned to take the times with a pinch of salt.
With dry conditions returning for qualifying, Max Verstappen took pole for Red Bull ahead of the Ferrari duo of Charles Leclerc and Carlos Sainz, while Hamilton and Russell could only finish sixth and eighth respectively.
Mercedes opted for a high-downforce set-up for Suzuka to cope with the tight first sector and the technical chicane to close out the lap, but Russell said it had left the team exposed when it came to the long straights.
"We've known this year we've not had the most efficient car, very draggy, and this is the first high downforce circuit where you have long straights as well," Russell explained. "Normally when you look at high downforce circuits, the straights are relatively small and there's not that chance to get those straight line deltas as well.
"I think we're probably losing seven or eight-tenths to Red Bull in the straights today, and the circuit has exposed that weakness of ours."
Mercedes ended qualifying as the fourth-fastest team as Alpine managed to qualify fifth and seventh with Esteban Ocon and Fernando Alonso, with Russell admitting the team "didn't expect it to be this difficult."
"We definitely didn't expect it to be in the fight for pole," said Russell.
"I think we've learned that by now, we need specific circuits to be in the fight for pole position. But certainly, we didn't expect to be that far behind the leader and that far behind the Alpines.
"But it's not the first time this season. There's quite a theme that we have difficult Saturdays."
Mercedes technical director Mike Elliott echoed Russell's comments, saying the simulations indicated the team would perform better than it did in qualifying.
"I think we're a bit disappointed with the performance, but we weren't expecting it to be a really good circuit for us," said Elliott.
"We expected it to be a bit more challenging. Where we want to run the car optimally in downforce level appears different to where others want to run, and that's part of the way we've developed the car and part of what we need to adjust over the winter."
Hamilton said the car was "feeling really good" despite finishing nine-tenths of a second off Verstappen's pole time, but he was sure that "three-quarters of that is just on the straights."
The seven-time world champion was open to rain hitting tomorrow's race, although he acknowledged it could be difficult for Mercedes to make progress without DRS in the wet.
"It's not going to be great with our car and no DRS," said Hamilton. "I think Red Bull is still quicker with no DRS on the straight than us when we have DRS open. But we'll be quick through the corners.
"The rain always opens up more opportunity. I think it could be not the most exciting race if it's dry. At least not for us, overtaking."
