Lewis Hamilton admitted that Mercedes was not where it wants to be versus its front-running rivals after the opening day of running at Sochi.
Hamilton concluded his day fourth fastest, a little over a tenth behind teammate Valtteri Bottas but 0.798s adrift from Red Bull's Max Verstappen, the afternoon session's pace-setter.
The reigning world champion said it had been "a day of discovery" for Mercedes during which the team's progress was matched by the improvements of its opponents.
"It's been a day of discovery and exploring. It started off not too bad and while we've been improving, the others made some big progressions and improved much faster than we did," he said.
"Our pace was a little bit better in FP2, but still not where we want to be. We'll work very late tonight to understand what we can do better and make some changes to the car for tomorrow.
"It's good to see the competition doing so well, but naturally we want to be in a fight with them, so that's what we're working towards."
Hamilton hoped the heavens would open above Sochi on Saturday to help level the playing field.
"It's going to be a tough day tomorrow, but I hope it's going to be raining," he added.
"I don't think we've ever driven here in the rain, so that would make it pretty interesting."
©Mercedes
Bottas confirmed that the Silver Arrows squad had work to do to put itself in the mix for qualifying, and he also hoped for a wet track come tomorrow afternoon's shootout.
"The balance of the car felt good, I was just struggling with the overall grip," commented the Finn.
"Everything else seemed ok, but obviously we need to find some gains if we want to be in the fight for pole tomorrow.
"We will review the data tonight to see what we can do better. Tyres are key for a good performance on this track and we need to try and extract more from the Soft tyres on a single lap, but our performance on the Medium and Hard tyre looked relatively decent.
"I hope for rain tomorrow, I think we might have a better chance in a wet qualifying."
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