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Red Bull's Max Verstappen edged Ferrari's Charles Leclerc in second practice at the season-opening Bahrain Grand Prix.
Ferrari's Carlos Sainz was third with Mercedes driver George Russell fourth, but 0.5 seconds off the pace.
Lewis Hamilton could manage only ninth fastest time in his Mercedes.
Verstappen was 0.084secs quicker than Leclerc as Red Bull and Ferrari seemed to establish themselves as the teams to beat.
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Fernando Alonso was fifth for Alpine, but nearly a second off the pace, and ahead of Alfa Romeo's Valtteri Bottas, Red Bull's Sergio Perez, Haas' Mick Schumacher, Hamilton and the second Haas of Kevin Magnussen.
McLaren driver Lando Norris was 11th fastest, team-mate Daniel Ricciardo down in 18th place after suffering from a water leak.
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Red Bull and Ferrari also appeared to be closely matched when the teams simulated race fuel loads later in the session, with Verstappen and Leclerc lapping at a similar pace, albeit on a very small sample of laps.
The evidence of the day seemed to confirm that Mercedes have been telling the truth in saying they are struggling for pace at the start of this season, in which Formula 1 has introduced the biggest regulation change for 40 years in an attempt to close up the field and make the racing more competitive.
The Mercedes car did not look comfortable to drive. The drivers were suffering from it "porpoising" on the straight - the consequence of an aerodynamic phenomenon to which these cars are prone in which the downforce level alternates at high speed, causing the car to bounce.
Hamilton was also complaining of problems with his front brakes and his DRS overtaking aid.
Russell's lap suggests the Mercedes have the potential to be third fastest team but they look to be a significant margin behind Red Bull and Ferrari, although a definitive picture will not be seen until after qualifying on Friday.
More to follow
The cars' front and rear wings have changed to smooth airflow