Brazilian Rubens Barrichello seized pole position on Saturday for his home Grand Prix in a nightmare scenario for championship-leading Brawn team mate Jenson Button.
Britain's Button, 14 points clear of the Sao Paulo driver with just two races remaining, qualified only 14th in a marathon rain-delayed qualifying session that ranked as one of Formula One's longest.
The third man in the title battle, Red Bull's Sebastian Vettel, is 16 points behind Button and will start in 16th place after being caught out by the weather.
Barrichello timed his first pole of the season, and first since the 2004 Brazilian Grand Prix, to perfection and is now well-placed to take the title battle down to the wire in Abu Dhabi.
The passionate local crowd, who had waited patiently through a rain delay of more than an hour and a half at the bowl-like Interlagos circuit, stood and roared in delight with cheers of 'Rubinho, Rubinho'.
Australian Mark Webber will share the front row with Barrichello in his Red Bull, who must finish one-two on Sunday to have any hope of preventing Brawn from winning the constructors' title.
VETTEL DEFIANT
"I'm keeping my feet on the ground because we have won nothing yet," said Barrichello, who has never won his home race despite twice previously starting on pole.
"I'm not watching what's going on on the side, just focusing. I will race as fast as I can to win the race and when its finished I'll get on the radio to see what happened to Jenson and Sebastian," added the 37-year-old.
Despite his speed, Barrichello only just made it through to the final phase of qualifying in 10th place while Button failed altogether and will line up in a risky position alongside Renault's error-prone rookie Romain Grosjean.
"(At) the start of the session, we had way too much understeer and I couldn't do anything with it and then on lap three, the rear started going away," Button told the BBC.
"We made a mistake not putting inters (intermediate tyres) on at the end. It's all of our decision, it's a team effort."
Vettel must finish second to stay in the hunt but he refused to give up his title hopes, vowing to race on Sunday "with the knife between our teeth".
"I think I went to the pool not the racetrack today. Obviously it's extremely disappointing. That's life," the 22-year-old German said.
Force India's Adrian Sutil qualified third with Italian Jarno Trulli fourth for Toyota. They will have Ferrari's Kimi Raikkonen and Toro Rosso's Swiss rookie Sebastien Buemi behind them on the third row.
Outgoing Formula One champion Lewis Hamilton and his McLaren team mate Heikki Kovalainen also failed to make it through the opening session, with the Briton qualifying 18th.
"The conditions were awful but, wow, our car was terrible," said Hamilton.
The first phase of qualifying was halted after four minutes when Italian Giancarlo Fisichella spun and stalled his Ferrari, with the car having to be lifted off the track with a crane. There was a 14-minute delay before qualifying resumed.
The start of the second phase was also delayed by heavy rain and, once it had started, stopped after two and a half minutes when Force India's Vitantonio Liuzzi crashed heavily at the end of the pit straight.
The impact, with the car hitting the tyre wall backwards, ripped both the rear wheels off but the Italian clambered out unhurt and waved to the crowd.
