Mercedes topped Friday's opening free practice session for the Russian Grand Prix, with Valtteri Bottas and Lewis Hamilton edging out Red Bull's Max Verstappen at the Sochi Autodrom.
Bottas posted a time of 1:34.427 on Pirelli's Soft compound tyre during the one-hour session, two-tenths faster than Hamilton and Verstappen managed.
Bottas has a strong record in Sochi, claiming his maiden F1 victory at the venue in 2017, securing pole position in 2018 (before being asked to move over for Hamilton) and triumphing again at last year's event.
Hamilton and Verstappen traded words in Thursday's media sessions, as their battle for the 2021 championship intensifies on and off the track, with just 0.016 seconds separating the pair at the end of FP1.
Verstappen will take a three-place grid drop for the clash with Hamilton last time out at Monza, while a second penalty is possible, should he and Red Bull decide to fit a new power unit this weekend.
One driver who has already taken on a new engine is Charles Leclerc, who wound up fourth for Ferrari, the only other driver within a second of Bottas' benchmark.
Result Free practice 1 - Russian
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There was a significant gap back to Sebastian Vettel in fifth, fresh from confirming his new deal with Aston Martin for 2022. AlphaTauri's Pierre Gasly, Ferrari's Carlos Sainz and McLaren's Lando Norris all followed closely, with the midfield battle again looking tight.
Norris experienced a strange moment on his way back to the pits at the end of the session, spinning at slow speed at the pit entry and clipping the wall. He expressed his confusion over the radio, asking if the rear wheels had locked up.
Sergio Perez was ninth in the other Red Bull, more than one-and-a-half seconds slower than teammate Verstappen, with Alpine driver Fernando Alonso completing the top 10 positions.
Alonso has expressed his fears about the weather forecast for the remainder of the weekend, given what happened at the Belgian Grand Prix, with heavy rain expected for Saturday's running.
Most drivers switched to longer runs for the second half of FP1, perhaps with the weather in mind.
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