Max Verstappen was dealt a blow in his bid to win Sunday's Emilia-Romagna Grand Prix after he broke down in practice.
The afternoon running in Imola for the second instalment of the championship was barely 10 minutes old when a mechanical failure ended Verstappen's participation.
The Dutchman, bidding to avenge defeat to Lewis Hamilton at the season-opening Bahrain Grand Prix last month, pulled over at the end of the pit straight before returning to the Red Bull garage, with his team reporting a suspected driveshaft failure.
Verstappen will be back for qualifying on Saturday, but the loss of track time could prove costly for the 23-year-old as he looks to take the title fight to Hamilton.
Hamilton opened his bid for an unprecedented eighth world championship with a defensive masterclass to keep Verstappen at bay and win in Bahrain.
But in terms of outright pace, Mercedes had few answers for Verstappen and Red Bull at the curtain raiser.
However, the black machines appear in better shape in Imola after completing a one-two finish in both of Friday's sessions.
Bottas ended the day just 0.010 seconds clear of Hamilton, with AlphaTauri's Pierre Gasly in third, 0.078 secs off the pace.
A widespread technical failure at Imola meant that drivers were cast adrift, unable to speak with their respective pit walls over the radio.
Indeed, the stewards determined that the communication woes had a role in Ocon and Perez's collision as the two made contact through the fifth corner.
Perez sustained a left-rear puncture while Ocon was also forced out with damage to the front right of his Alpine but both men returned for the second session.
Mazepin was also able to take part in the concluding action after he crashed out at the final corner in the closing moments of the opening running.
With just three minutes remaining, the Monegasque lost control of his Ferrari through the left-handed turn 18 before running over the gravel and into the wall.
"I am sorry, guys," Leclerc said over the radio. "I was pushing quite a bit."
Perez finished sixth in his Red Bull, almost nine tenths slower than Bottas. McLaren's Lando Norris was ninth as Sebastian Vettel's troubled start to life with Aston Martin continued after he finished only 15th.