Rain threatened to put a dampener on the Barcelona test early on Saturday, but the 2010 pecking order was also clouded.
"P1 today is nice but means nothing, to be honest," Williams' Nico Hulkenberg said after topping Friday's timesheets.
Also quick on Friday was Ferrari, while McLaren and Mercedes seemed to be struggling for pace, and Sebastian Vettel in the Red Bull could manage just the fifth fastest time.
"I don't know what was going on with McLaren and Mercedes today. I haven't looked that closely but they didn't seem to be that quick," said German Vettel.
Demonstrating how fickle are the developing conclusions about the pecking order is that Mercedes' Nico Rosberg had matched Friday's pace before the rain began to fall on Saturday.
But after Friday's running, Michael Schumacher had admitted the W01 might not be a winner.
"Maybe not," the seven time world champion told reporters when asked if he would be among the favourites in Bahrain.
McLaren's Lewis Hamilton took the contrary position, arguing that suggestions his car is off the pace are wrong.
"We're in a much stronger position than we were last year," he said. "After driving last year's car here this feels light years ahead."