Red Bull's Sergio Perez and Alpine's Esteban Ocon tangled in practice for the Emilia Romagna Grand Prix at Italy's Imola circuit on Friday, bringing out red flags before Mercedes emerged on top.
The opening session ended with Valtteri Bottas quickest in one minute 16.564 seconds and the Finn's seven-times world champion team mate Lewis Hamilton, last year's race winner, a mere 0.041 slower.
Red Bull's Max Verstappen, Hamilton's closest rival in the Bahrain season-opener when he started on pole and finished second, was third on the timesheets and 0.058 off the pace.
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The session ended with Russian rookie Nikita Mazepin spinning his Haas at the final corner for the second time in the space of an hour.
Ferrari's Charles Leclerc was fourth fastest with AlphaTauri's Pierre Gasly fifth and Ferrari's Carlos Sainz sixth.
The turn five incident between Frenchman Ocon and Mexican Perez, former team mates at the Force India team that is now Aston Martin, was unclear due to a television technical problem.
It appeared there had been a coming together, however.
"It sounds like Ocon was on a slow lap, Sergio was on a flying lap and there's been contact between the two guys obviously into that corner," said Red Bull principal Christian Horner.

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"I haven't seen a replay, I've only got it second hand from the driver."
The technical problems meant teams were without some images and timing data.
Perez's car came to halt with the left rear tyre in shreds while Ocon parked up further down the track.
Both drivers and team representatives were summoned to the stewards while mechanics worked to repair the damage in time for the second session.
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