Sergio Perez went quickest in Friday's morning practice session ahead of Formula One's Italian Grand Prix at Monza, while Ferrari's Sebastian Vettel was quickest in the afternoon.
Wet conditions affected the session early on, but the Force India man took advantage of the drying track to set a time of one minute, 34 seconds in the final lap of the session.
Kimi Raikkonen finished second fastest, 0.550 seconds behind the Mexican, while Esteban Ocon rounded out the top three.
The track dried out in the afternoon session which was disrupted by a dramatic high-speed crash involving Marcus Ericsson. The Sauber driver escaped unhurt after hitting the barrier and flipping over several times.
Channel 4 F1 showed just how serious the incident was:
The session was red flagged for 20 minutes which means teams had to adjust their plans.
When practice resumed it was Vettel who came out on top ahead of team-mate Raikkonen.
Here's the timesheet from the afternoon session:
Vettel finished the day fastest despite a spin into the tyre barrier at Parabolica:
The German, who is 17 points behind leader Lewis Hamilton in the drivers' championship, once again showed his pace to take top spot.
However, the session was largely overshadowed by Ericsson's crash. Sky Sports F1 shared footage of the accident:
It was yet another serious incident following on from Charles Leclerc's crash at the Belgium Grand Prix on Sunday.
The incident could have been caused by the DRS still being open, according to BBC Sport's Andrew Benson:
When the session resumed Vettel impressed, while Hamilton made errors on his fastest laps and could only finish third.
Ferrari look in good shape for the race ahead of Saturday's qualifying session, as Vettel aims to continue to cut the gap to Hamilton.
Here is the classification from FP1:
Daniel Ricciardo led the field out onto the track at the start of the session, but just moments in the Australian lost power:
The Red Bull driver finished the morning strongly, though, to secure P5.
Due to the conditions, Nico Hulkenberg was the only driver to set a time in the first 25 minutes, before his Renault team-mate Carlos Sainz got his name on the timesheet.
Times began to pile up as the track dried, though Max Verstappen did spin at Parabolica:
Mercedes pair Lewis Hamilton and Valtteri Bottas both took turns atop the timesheet, but they were soon replaced by Verstappen following his spin.
Sebastian Vettel's session came to an early end as the German required a gearbox replacement on his Ferrari, while Mercedes also brought their efforts to a premature close.
Their absence, coupled with a rapidly drying track, prompted a frenetic end to the session as Raikkonen and Hartley took turns at P1 before Perez secured it with his last-ditch effort.
If conditions continue to improve as expected, the times could be even quicker in the afternoon run.