Red Bull's Daniel Ricciardo beat Mercedes and Ferrari to set the fastest time in first practice at the Bahrain Grand Prix.
The Australian was 0.304 seconds ahead of Mercedes' Valtteri Bottas, who edged Kimi Raikkonen's Ferrari by 0.094secs.
World champion Lewis Hamilton was only fifth quickest, behind Ferrari's Sebastian Vettel, after making a mistake on his flying lap on the fastest tyre.
Haas' Romain Grosjean was best of the rest, ahead of Toro Rosso's Pierre Gasly.
McLaren, who have introduced some of the aerodynamic upgrades that were initially planned for the first race of the season, had a difficult session, Fernando Alonso only 11th quickest.
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Ricciardo's team-mate Max Verstappen suffered an electrical problem on his first lap and missed the rest of the session.
Behind Gasly, the top 10 was completed by Renault's Carlos Sainz, who was separated from his team-mate Nico Hulkenberg in 10th by the second Haas of Kevin Magnussen.
Hamilton heads into the Bahrain weekend intending to make amends for the Mercedes strategic error that surrendered victory in the season-opening race in Australia to Vettel.
The German said on Thursday that he believed Mercedes had an advantage of about 0.3-0.4secs over the field but Hamilton insisted that the two leading teams were just as close as they were for most of 2017, when Hamilton and Vettel staged a very tight battle for the title until Ferrari's season imploded over three races in Asia in September and October.
Ricciardo, who finished fourth in Australia right behind Raikkonen, said he was optimistic Red Bull could be in the fight for victory if the car was as good as it was in the race in Melbourne.
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