Nico Rosberg fears that the KERS-equipped McLarens and Ferraris could wreck his race in the European Grand Prix.
Although confident of being able to fight for a top six place on pace after a promising first day of practice, the German is wary of the threat posed by the KERS cars even if he is able to outqualify them.
"We can be happy with another top five this weekend, but a top six is more realistic because this is a KERS track," said Rosberg when asked by AUTOSPORT what his target was for the weekend.
"If the KERS cars are right behind me in qualifying, it's very likely that they will be in front of me at the end of the first lap. Then, even if they are slower, they are going to finish ahead in the race.
"It' s going to be very important to figure out where the KERS cars are going to be because you know that they can attack at the start. When you are stuck behind them, it can all go wrong.
"You either need to do what Fernando Alonso did in Hungary and be far ahead on a three-stop or try to beat them some other way."
Rosberg admitted that he was surprised by the pace shown by Alonso's Renault on Friday after the Spaniard outpaced the rest of the field by three -quarters of a second.
"We made some good progress with the setup, but I must say that Alonso's time was a bit surprising so that has made us a bit less confident. But compared to the others, we seem to be OK.
"The whole first session I had tyre graining so that was a waste of time. Then in the afternoon we progressed on setup and in general we have made a step forward."