Lewis Hamilton headed championship rival Nico Rosberg in first practice at the Mexican Grand Prix.
Hamilton was 0.759 seconds ahead of Rosberg, exactly the sort of display he needs at a race he really needs to win to keep his title hopes alive.
Rosberg, who leads by 26 points with 75 points available from three races, will be crowned champion on Sunday if he wins and Hamilton is 10th or lower.
Hamilton was fastest and Rosberg only seventh in an inconclusive session.
On a chilly morning in Mexico City, with temperatures at the start of practice only 10C, the two Mercedes drivers set their best times on medium tyres, while the four drivers between them were on softer rubber.
Ferrari's Sebastian Vettel was second fastest, 0.079secs behind Hamilton and just ahead of team-mate Kimi Raikkonen, both using the soft tyre, and the Force Indias of local hero Sergio Perez and Nico Hulkenberg next up on super-softs.
Williams's Valtteri Bottas was sixth, also using the softs.
It was an inconclusive session as a result of the different tyre choices and the constantly changing track surface at the Autodromo Hermanos Rodriguez, which started off extremely dusty and became quicker through the session as the circuit cleaned up.
But Mercedes did say that Rosberg had not had any problems, and the two Mercedes drivers did set their times at similar points in the session, when the difference in track surface would have been minimal.
The dirty track led to a number of drivers locking their wheels and running wide, but there was a bigger concern for Red Bull, where Max Verstappen suffered a rear brake fire midway through the session which damaged sensors and brought his running to an end.
The Dutchman was 14th fastest, with team-mate Daniel Ricciardo, who needs only one point this weekend to secure third place in the championship, eighth.
Sauber also had a difficult session after Felipe Nasr suffered a front wing failure after 35 minutes, the part exploding on the straight after he had bounced over a kerb on the exit of the Esses.
The wing sheered along the line where the FIA-mandated neutral section in the centre of the wing meets the part at which the teams can start playing with the design.
Sauber also felt another part of the car had failed at the same time and stopped running for much of the rest of there session, with the car of Nasr's team-mate Marcus Ericsson also held in the pits.
Brazilian Nasr was sent out in the closing minutes, however, once the cars had been checked and split the McLarens in 17th place, while the Swede stayed in the pits.
Jenson Button was only 18th for McLaren, two positions and 0.253secs behind team-mate Fernando Alonso, both using the mediums, on a weekend when both expect to struggle for competitiveness because of the Honda engine's lack of power on the long straights.
Also on mediums, Jolyon Palmer was 21st, three places and 0.541secs adrift of team-mate Kevin Magnussen as both seek to convince Renault that they deserve a seat at the team alongside new signing Hulkenberg in 2017.