McLaren's Jenson Button beat Red Bull's Sebastian Vettel to set fastest time in final practice at the season-ending Brazilian Grand Prix.
Vettel's title rival Fernando Alonso is struggling, setting only eighth fastest time, 0.503 seconds off the pace.
Red Bull's Mark Webber was third from McLaren's Lewis Hamilton, with Lotus's Romain Grosjean fifth.
Alonso is 13 points behind Vettel and must finish on the podium to have any chance of overhauling the German.
The Spaniard must win with Vettel lower than fourth, be second with Vettel lower than seventh or be third with the Red Bull driver below ninth to win the championship.
Alonso was not only slower than the McLarens, the Red Bulls and Grosjean, but was also behind both Force India cars, with Paul di Resta just edging ahead of Nico Hulkenberg to take seventh.
Grosjean's team-mate Kimi Raikkonen suffered an engine failure and failed to set a lap time in Saturday's final practice session, in which the teams simulate their cars in qualifying trim.
Although which he aborted after being delayed in the final corner by a slower car, the Ferrari driver faces the very real prospect of qualifying ninth, from where overhauling Vettel would be an even taller order than it already is.
"The Red Bulls and the McLarens brought good developments to the last race in Austin," BBC F1 technical analyst Gary Anderson said.
"Ferrari have brought some stuff but they've just tidied up the car, so they haven't really gone forward and they're paying the price for that."
However, rain is forecast for the afternoon and there are some predictions that it could arrive before the end of qualifying.
In the first part of the session, with the drivers running on the slower 'hard' tyre, Button was fastest from Vettel, Hamilton, Massa and Webber.
Alonso was only sixth, 0.498secs off the pace, and 0.301secs behind Massa, who until the last laps in final practice has looked the quicker of the Ferrari drivers this weekend.
In a generally incident-free session, Williams driver Bruno Senna spun coming up the hill out of the Juncao corner, in a very similar place to where his uncle Ayrton Senna spun into retirement while trying to chase Michael Schumacher for victory in the 1994 race here.
The German was the next car on the scene and narrowly avoided colliding with the Brazilian, who is fighting for his F1 career with the Williams reserve driver Valtteri Bottas expected to take his seat in 2013.
Final Practice top 10
1. Jenson Button - McLaren 1:13.188
2. Sebastian Vettel - Red Bull +0.057
3. Mark Webber - Red Bull +0.197
4. Lewis Hamilton - McLaren +0.201
5. Romain Grosjean - Lotus +0.232
6. Paul di Resta - Force India +0.298
7. Niko Hulkenberg - Force India +0.414
8. Fernando Alonso - Ferrari +0.503
9. Pastor Maldonado - Williams +0.512
10. Felipe Massa - Ferrari +0.524