Michael Schumacher admits that he is "slightly concerned" about the pace of the new Mercedes so far in testing.
The German legend, 42, told BBC Sport in an exclusive interview that it was important for Mercedes to improve on a disappointing 2010 campaign.
But he said it would be "difficult" for Mercedes to win races at the start of the season next month.
"Yes, we are slightly concerned but then on the other hand we know where we will and want to go to," he said.
"The first race will tell us truly what is the situation."
The season starts in Bahrain on 13 March and Formula 1 teams are currently in the middle of their pre-season testing programme.
And he was third fastest overall on Saturday, behind Nick Heidfeld in the Renault and Fernando Alonso's Ferrari.
Schumacher said he had not yet had enough running to know how well the Mercedes was in tune with them.
"We have a character of car that we built up," he said, "but we have a new character of tyres and how much they will be in harmony is difficult to say at this moment in time.
"It seemed less important last year and is more important this year."
