The long straights in the opening sector are providing Ferrari with rapid sector times compared to rivals Mercedes, who in turn have an advantage in the second sector.
Vettel doesn't think the margin is representative and has warned Ferrari about not getting complacent with its position on the timesheet.
"I don't think the margin is correct. I don't know what the others are doing but we've seen a few times that we're quick on a Friday, so I'm not blinded by that.
"It's okay, I think it can be a bit better so we're working on it. It's a Friday, the reality is that it's a Friday. We have to wait until tomorrow night and especially Sunday when it matters.
"We're less competitive in sector two and more in sector three where there is a straight. We knew [that], but we have to work on sector two, work on the feel of the car there."
For the Grand Prix on Sunday, the temperatures are expected to drop by a number of degrees compared to Friday.
However, it doesn't concern Vettel despite the issues Ferrari has had this season with the Pirelli tyres and switching them on properly.
"Not concerned, it's a drop for everyone. It will be difficult to carry today to Sunday, but it was still useful to do the laps and we've got some homework."
Bahrain International Circuit - Winter testing
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