Nico Rosberg beat Mercedes team-mate and title rival Lewis Hamilton in both Friday practice sessions at the Brazilian Grand Prix.
The German was 0.221 seconds quicker than Hamilton in the first session and 0.213secs ahead in the second.
Hamilton, 24 points clear of Rosberg with 75 points available in the final two races, compromised his session with an off-track moment.
Ferrari's Kimi Raikkonen was third ahead of Red Bull's Daniel Ricciardo.
In the second session, held in temperatures of 29C as forecast rain held off, Hamilton was faster early on with his slower 'medium' tyres.
But on the 'soft' tyres, when simulating qualifying pace, he made a mistake at the fast, downhill Mergulho corner and was 0.599secs slower than Rosberg.
A second lap when the tyres were past their best moved Hamilton to within 0.213secs and suggested he could at least compete on pure pace.
"The car feels OK," said Hamilton. "It wasn't the best of days. I didn't really put together a good lap but that will come tomorrow hopefully."
Hamilton has a comfortable lead in the championship but the spectre of double points at the final race of the season in Abu Dhabi in two weeks' time means the championship is still wide open.
Hamilton, who has won 10 races this season to Rosberg's four, could win in Brazil and still lose the championship to his team-mate if he hits reliability problems in Abu Dhabi.
Already they seem poised for another private battle for the lead, although predicted wet weather for the weekend adds an element of uncertainty.
The two were well ahead of the rest of the field - Raikkonen was 0.573secs adrift of Rosberg and was suspected to be running less fuel, as Ferrari tend to do on Fridays.
But the Finn, who was testing a new rear suspension design with a view to 2015, had the rare boost of being faster than team-mate Fernando Alonso, who was 0.426secs off Raikkonen and only seventh fastest.
Alonso later had an engine failure, pulling off on the back straight with flames pouring from the exhausts.
If Alonso has to take a sixth engine for the season he would incur a grid penalty but a Ferrari spokesman said that "at the moment that is not the plan".
|Hamilton v Rosberg 2014 title permutations|
|Hamilton has a 24-point lead over Rosberg with a maximum of 75 points remaining - 25 in Brazil for a win and 50 in Abu Dhabi.|
|The 2014 Formula 1 world title cannot be decided at this weekend's Brazilian Grand Prix.|
|If Hamilton wins in Brazil and Rosberg fails to finish, he will have a 49-point lead going to Abu Dhabi. Hamilton would then need to finish 10th or higher to clinch the title.|
|If Rosberg wins in Brazil and Hamilton retires, Rosberg will lead the championship by one point going to Abu Dhabi. Hamilton would then need to finish ahead of Rosberg in the points in Abu Dhabi to secure the title.|
|If Hamilton wins in Brazil, with Rosberg second, Hamilton will arrive in Abu Dhabi with a 31-point lead. But if Rosberg then wins and Hamilton finishes sixth or lower, Rosberg will win the title.|
The session was halted for the Ferrari to be recovered and as soon as it resumed it had to be stopped again as Esteban Gutierrez's Sauber came to a stop.
Raikkonen had an off-track moment at the Juncao corner leading on to the long main straight, after three spins in the morning session.
The two Ferraris were separated by the Williams cars of Valtteri Bottas and Felipe Massa.
Toro Rosso's Daniil Kvyat was eighth, ahead of Red Bull's Sebastian Vettel, the man the Russian will replace in 2015 when the world champion moves to Ferrari.
McLaren's Kevin Magnussen was 10th fastest with team-mate Jenson button in 16th and unhappy with the handling of his car.