Mercedes driver George Russell has said the opening days of preseason testing suggest rivals Red Bull remain a step ahead of the rest of the Formula One field.
Reigning champion Max Verstappen topped the opening day of this week's test by over a second, albeit with more attempts at a fast lap than his rivals and when track conditions were at their best in the final hour of the session.
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Russell said Red Bull's ability to hit the ground running came as no surprise following the team's dominance in last year's championship and he expects them to continue as the team to beat in Formula One when the season gets underway on March 2.
"I think it was probably as you'd expect from Red Bull, they're in such great momentum at the moment," Russell said. "They started these new regulations on the front foot. They came here and have been solid ever since.
"As I said, they've been the favourites, they're a step ahead of everybody else here in Bahrain. They've had an impressive winter no doubt.
"But that was to be expected, such an impressive team and outfit, and we need to see where we fall out in the coming races."
Russell was 12th fastest on the opening day of testing as he focused on refining Mercedes' setup and completing a race simulation towards the end of the afternoon session.
Mercedes has changed its car concept this year while targeting improvements to its drivability after both Russell and teammate Lewis Hamilton said they could not trust the handling of last year's W14.
"It definitely is an improvement, there's no doubt about that," Russell said of the new W15. "You can have the worst car to drive, but if it's faster than everybody else, you would be happy with it.
"It was definitely pleasurable to drive yesterday, I had a good feeling within the car, but all the other teams made a good step forward and right now, it's definitely far too early to say.
"We have a huge mountain to climb to catch up what Red Bull were doing last year, how far ahead of everybody else they were. We need to wait and see, but we've definitely got a much better platform to build upon. It's not the diva that it was the past two years."
Asked where the car still needed improvement to match Red Bull, Russell added: "More downforce. When the car feels nice but it's not quite on the pace, you just need to find downforce in the right places.
"This iteration of regulations, there's definitely a sweet spot for all of the teams. You definitely want the car as low as possible, but you can't go too low in case you're bottoming out.
"It doesn't take a lot to find that sweet spot and find a lot of performance. And hopefully Red Bull are already in that sweet spot and we can close that gap [by finding it], but yeah, it's going to take a lot of hard work to do so."