A car upgrade may have allowed Mercedes to catch up with Ferrari, according to Sebastian Vettel.
After four consecutive pole positions for Charles Leclerc, Mercedes' championship leader Lewis Hamilton said recently that Ferrari had "changed something about the engine" that makes it "better than all the others".
He therefore urged Mercedes to resume its 2019 development race, and the Brackley based team duly arrived at Suzuka with updates around the bargeboards, front wing and sidepods.
In Friday practice, Mercedes was clearly the fastest team, with Ferrari struggling even to keep up with Red Bull's Max Verstappen.
"This is the same car, I assure you," Vettel said at Suzuka.
"Maybe the other cars have changed - I don't know. I think they have probably improved, but it's also possible that this track just suits Mercedes better."
Leclerc, who has been on pole for the last four races, agreed: "Yes, we are definitely lagging behind so far, and Red Bull is pretty fast too."
And he doubts that what Hamilton referred to recently as Ferrari's qualifying 'jet mode' will be enough to speed up in qualifying on Sunday morning.
"I don't think so," he said. "Mercedes is just very fast this weekend.
"In the second practice, everyone used their qualifying mode because there is no certainty that we can qualify on Sunday morning.
"That's why I think the results we see roughly reflect the real balance of power."