Jenson Button's hopes of wrapping up the world championship in Brazil were dealt a serious blow after the Brawn driver qualified a lowly 14th for Sunday's race at Interlagos.
The 29-year-old Briton admitted it was a mistake not to switch to intermediate tyres at the end of Q2, despite several of his rivals doing so.
He then found himself unable to responds as other drivers moved ahead of him after a protracted red flag period held up proceedings in the rain-interrupted season.
"At the start of the session I had way too much understeer in the car on that run, when the circuit wasn't wet like it was in the first session," Button told the BBC.
"I couldn't do anything with the car and on lap three the rears started going away so that was it.
"We made a mistake not putting the inters on at the end of the session."
Button, whose team-mate Rubens Barrichello grabbed pole position, refused to apportion blame for the decision when asked.
"Well it's all of ours, it's a team effort isn't it," he said.
Button starts the grand prix two positions higher than his other championship rival Sebastian Vettel, who was knocked out in Q1.