Lewis Hamilton fastest in Formula 1's Australian GP practice
Lewis Hamilton started the new Formula 1 season with fastest time in the first practice session at the Australian Grand Prix.
The world champion had emphasised on Thursday that he felt Mercedes were behind Ferrari heading into the season.
But in a closely matched session, Hamilton edged Ferrari's Sebastian Vettel by 0.038 seconds. His team-mate Charles Leclerc was 0.036secs behind.
Just 0.193secs covered the top four, with Red Bull's Max Verstappen next up.
British-born Thai Alexander Albon, a rookie driving for Toro Rosso, brought the session to a brief halt with a crash at Turn Two.
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The incident knocked off the front wing but Albon was able to drive the car back to the pits.
The other new drivers, Britain's Lando Norris and George Russell, began cautiously on the Albert Park street track, which is notorious for its dirt and bumps and where it is easy to lose the car and damage it.
Norris ended the session 18th, four places and 0.6secs off team-mate Carlos Sainz. Russell was last and 0.8secs off his Williams team-mate Robert Kubica.
Williams were, as expected, adrift of the back of the field - Kubica was two seconds off the next slowest car, which was Norris.
Part of that margin was in tyres - Williams only ran the medium tyre while most others set their times on the quickest soft - but that still leaves the Williams more than a second off the back of the grid.
At the front, Hamilton's performance undermined Mercedes' claims that they were behind Ferrari.
The world champions led the way throughout the session, Bottas starting off with the quickest time when running the medium tyre, before Hamilton vaulted ahead by 0.267secs on the soft.
Bottas ran wide at the penultimate corner and narrowly avoided clouting the wall while trying to improve his time.
Sixth quickest was Alfa Romeo's Kimi Raikkonen, ahead of an impressive performance from the returning Daniil Kvyat in seventh place in the Toro Rosso on the medium tyre.
The second Red Bull driver Pierre Gasly was eighth quickest, ahead of Renault's Nico Hulkenberg, who missed the first half of the session with a technical problem.
His team-mate Daniel Ricciardo was baulked by Haas driver Romain Grosjean on his quickest lap so the Australian's 17th quickest time was not representative.