After setting pole and leading in both Bahrain and Australia, Sebastian Vettel said he is not "100 per cent happy" after Friday practice for the third round of the world championship.
"I think we still need to get a little bit quicker. The McLarens and the Mercedes look quick and Ferrari have a decent long run pace, so we'll have to see where we are," said the Red Bull driver at Sepang.
Teammate Mark Webber, who lost time in the second session with a Renault engine failure, said the RB6 felt "pretty good".
McLaren's Lewis Hamilton led both 90 minute practice sessions in hot and humid Malaysia, and the grey Mercedes cars also looked quick.
Said Hamilton: "We don't yet know where the others are but we're optimistic that we've made a small step forward."
Michael Schumacher said he thinks the W01 is "lacking a little compared to the front runners".
It was a slightly anonymous Friday for Ferrari, and Fernando Alonso was neither optimistic nor pessimistic, admitting he has "no idea of where we are".
"I'm seventh, but I could just as well have been second or last and it would not have changed anything," said the Spaniard.
His teammate Felipe Massa added: "What is clear is that Red Bull and McLaren seem to be going very well."
Meanwhile, Sauber ran its modified 'F-duct' system during Friday practice, driven to a best of 11th aboard Kamui Kobayashi's car in the second session.
"We will analyse the data here at the track and in the factory in Hinwil to decide on the next steps," said technical boss Willy Rampf.
But team principal Peter Sauber told Speedweek: "We are too slow and I see no reason why we should be more competitive here than in the first two weekends."