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 was third on the timesheets and 0.058 off the pace.
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.
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Technical problems that affected radio communications and data contributed to the turn five collision between Frenchman Ocon and Mexican Perez, former team mates at the Force India team that is now Aston Martin.
The coming together had been unclear immediately, with cameras not capturing images, but stewards decided to take no further action.
"Ocon slowed before turn five in order to let Perez by, as well as moving entirely to the inside of the turn," they said in a statement.
"Perez was on a fast lap. The two cars reached the apex at the same time, and just barely touched wheels.
"In the end, both drivers agreed in the hearing that it was an unfortunate miscommunication of timing between them, not helped by the fact that both drivers had limited or no communications with their teams at the time."
Perez's car came to halt with the left rear tyre in shreds while Ocon parked up further down the track.