Britain's Lewis Hamilton says Mercedes' tyre issues in race conditions are not over despite team-mate Nico Rosberg winning the Monaco Grand Prix.
While the car has had impressive speed over one lap in qualifying this year, race pace has been its big weakness.
Hamilton, who finished fourth, said: "No, we haven't sorted Sunday. It's still a bit of an issue for us.
"This is a track that seems to work better on tyres. When we go to the next one, who knows what's going to happen?"
Mercedes, who are the subject of a protest by Ferrari and Red Bull, who accused the team of doing an illegal tyre test in the run-up to the Monaco GP, saw Rosberg and Hamilton line up on the front row in Monte Carlo.
The duo ran one-two in the early stages before the safety car came out as a result of Ferrari driver Felipe Massa's accident at Sainte Devote.
Rosberg pitted and managed to retain the lead, but Hamilton dropped to fourth, behind the Red Bulls of Sebastian Vettel and Mark Webber.
"Without the safety car, we could have had a one-two," said Hamilton. "We were a bit unfortunate. When you go backwards, it's never satisfying.
"I slowed the car down too much when the safety car was out so when I came round to the pits, Nico had already done his stop. It was my fault.
"I was told to have a six second gap, maybe I had a bit more, and I lost out massively.
"Congratulations to Nico, he did a great job. He was much more on it all weekend. I've just got to keep working harder. I wasn't good enough."
Rosberg, who won the Monaco Grand Prix 30 years after his father Keke triumphed in the same race, was delighted with the win and hopeful for the rest of the season.
"It's the grand prix to win if there's one you have to choose from," said Rosberg, 27. "Winning the Monaco Grand Prix, in a silver arrow, as a German - it doesn't get much better than that.
"It's coming together, slowly but surely, but we have got to remember just recently we've been struggling and this track suits us.
"We've improved a little bit but we still have some work to do. We're not the best yet, we're still quite far away and there's a lot of work ahead of us.
"But the team have done a great job - we've been able to keep up in the development race up to now."