Mercedes and Lewis Hamilton say it's possible Ferrari and Red Bull are now ahead having developed at a faster rate.
The Brackley-based team has not won since the summer break and would go their longest run without a victory in the hybrid era if they lose again this weekend at the Russian Grand Prix.
On Friday, the early signs weren't too positive either with Bottas six-tenths down on pacesetter Max Verstappen with the five-time Formula 1 champion almost two-tenths further back.
"It's been a day of discovery and exploring," said Hamilton via Sky Sports. "It started off not too bad and then it just didn't really improve as with the others, who made some big progressions.
"We anticipate that we're losing eight-tenths of a second to the Ferraris or something on the straights. Nonetheless, we've been improving but they are improving at a serious rate.
"We've just been figuring out how we can improve the car, but it's not an easy task."
Mercedes have a perfect record in Sochi having never been beaten in the past five visits to the Olympic Park, but Hamilton is unsure if that run will continue.
"That is the million-dollar question. I don't know," he said on if Mercedes can close the gap.
"Naturally, I believe in my guys. I don't think we're getting the maximum, maximum out of our car and that's probably been the key issue we've had for the last three races.
"We're still trying to discover it, but it's not easy out there. It's good to see the competition doing so well but naturally, we want to be in the fight with them so that's what we're working towards."
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Team boss Toto Wolff was equally unsure about Mercedes' chances.
"A negative day, to put it bluntly," he said of Friday. "We are just off the pace.
"We are struggling to find the right clues at the moment. I think we haven't really improved.
"Obviously we didn't put a lot of effort into developing the car and that might be a reason also. It seems that others have disproportionately gained and we have lost."
