Valtteri Bottas says Mercedes needs an upgrade to solve the handling issues discovered during the first run of its new 2019 Formula 1 challenger in testing at Barcelona last week.
While Ferrari enjoyed a strong first week of pre-season testing, Mercedes had a low-key start that suggested it is currently on the back foot.
Speaking at an event with Mercedes title sponsor Petronas in Italy, Bottas revealed that the main issue for the team was finding a consistent balance for the car over a whole lap and that the W10 has "massive" problems in some corners.
"We have work to do, but it's a good feeling in the sense that it feels like there is potential," said Bottas. "We do need to make improvements.
"We were struggling a bit over the week to find a good balance for all the corners.
"There were some corners that were good, some corners we would have balance issues and other corners [where] we would have massive balance issues.
"We were getting to a much better state at the end of the week, but some of the handling issues can only be sorted with some upgrades. We're hoping to fix those hopefully soon."
Lewis Hamilton said he had not wasted too much time worrying about what other teams had done last week, but had no doubt about the scale of the fight Mercedes faces.
"At the moment, we are behind," he said. "So, we don't approach the first race currently at all relaxed, not that we ever do. We still have a lot of work to do.
"This is going to be the hardest year yet, but I feel I have the best group of people behind to climb that mountain together.
"We are going to face our toughest challenge but I feel like I have the right soldiers behind me."
Mercedes boss Toto Wolff agreed that Ferrari is currently ahead, and reckons that the Italian team still has plenty of performance it is yet to show.
"In Barcelona [test] one what we have seen is that Ferrari is ahead of the pack," he said. "The car looks fast and is fast on lap times.
"[On Thursday] they could probably have gone 1.5 seconds faster if they wanted to, and that is the fastest car we have seen in testing so far.
"Then it is pretty much everybody bunched up, which is pretty exciting between all the other teams.
"Now it is about seeing what happens in Barcelona [test] two.
"I have a feeling everybody will bring stuff to the car and then again in Melbourne. So probably only then will you be able to judge the competitive balance and not before."