F1 world champion Lewis Hamilton has warned Mercedes are on the backfoot when they start their title defence in this weekend's season-opening Australian GP.
After an ultra-impressive winter, Ferrari are expected to be the quickest team when the new season begins in Melbourne.
"We have work to do," Hamilton admitted.
Mercedes struggled to keep pace with Ferrari in winter testing, and although the pre-season action has a reputation for delivering misleading clues as to the real pecking order, Hamilton added: "It wasn't difficult to read, it was clear."
But Hamilton later told Sky Sports: "It might be closer than we think, it might be a bigger gap than we think. We don't know what they've got up their sleeve."
Some paddock analysts believe Ferrari, who will pair Charles Leclerc with Sebastian Vettel this year, could be as much as four-tenths of a second ahead for the Albert Park curtain-raiser.
"At this point, we are all hunters and we are all hunted," said Vettel. "Hopefully going away from here we will be the hunted.
"Last year's winter testing didn't go well. This year we are more prepared, our car seems to work fine."
He continued: "We'll obviously try to have a great, smooth weekend. Its always nice to kick off the season with a good result."
But Hamilton's Mercedes team-mate Valtteri Bottas countered: "As a team we've learned a lot from the package we had in testing and how we can optimise it better for the first race. I feel we have made improvements already and I'm confident we can be in a better shape.
"It's only guessing where we are going to be but, as we last were on track, we are chasing Ferrari and we'll see where we are here."
Speaking at a press conference, the assembled drivers all paid an emotional tribute to FIA Race Director Charlie Whiting, who passed away on Thursday night.
F1's waiting game for the pecking order to be revealed
While Ferrari are predicted to be the early-season frontrunners, the rest of the pecking order is far less well defined.
Red Bull, who have switched to Honda power, are thought to be in close proximity to Mercedes while Alfa Romeo and Haas may well challenge Renault to be the 'best of the rest'.
"I'm excited to see the pecking order," said Daniel Ricciardo ahead of his debut for Renault. "We could draw some conclusions from testing but not much.
"In the second week, we had some new parts which seemed more exciting for our development. But judging the top three cars, I don't even know where they stand."
Red Bull have been encouraged by the first steps they have taken with Honda, whose engine ran faultless during testing.
"So far, it's been good and I was very happy by how everyone has been working together," said Max Verstappen. "In terms of performance, it's difficult to judge where we are."
The first practice session of the year begins at 1am UK time on Friday morning, with Sunday's race, which will be exclusively live on Sky Sports F1, underway at 5.10am.