Fernando Alonso tamed tricky conditions to finish fastest in first practice for the return of the Japanese Grand Prix.
The Alpine driver posted a lap time of one minute 42.248secs on the intermediate tyres, three tenths faster than Ferrari's Carlos Sainz around a sodden Suzuka circuit.
Charles Leclerc wound up third, a further 0.071s down on Sainz as drivers felt their way around during the first track running at the venue since 2019.
Rain fell heavily around half an hour prior to the session, with any action looking unlikely with rivers forming around the track.
But with the track surface draining nicely, Kevin Magnussen was the first driver to brave the conditions on wet tyres, joined by home favourite Yuki Tsunoda, team-mate Mick Schumacher and the Williams of Nicholas Latifi.
However, it wouldn't be until the Ferrari duo took to the track almost 20 minutes into the session that lap times would be posted.
Sainz warned of "a bit of noise from the engine", prompting minor concern for the Scuderia, though the Spaniard was able to continue unaffected.
McLaren's Lando Norris was another driver with a power unit issue, pitting at the halfway mark after being given a warning on his steering wheel.
Latifi was the first driver to make an error when locking up at the hairpin, before Daniel Ricciardo made a similar mistake at the final chicane.
Rain fell in the final 20 minutes and as drivers pressed on with intermediate tyres, Sainz went wide at Spoon before Leclerc twice had moments when hitting the inside kerb at Degner.
Red Bull was the first team to take to the intermediate tyres with 25 minutes to go in the session, Max Verstappen initially lapping 3.699s faster than Sainz's best extreme wet lap time.
But Alonso found the best speed on the green side-walled tyre, demoting his fellow countryman to second.
Leclerc was third ahead of the second Alpine of Esteban Ocon, with the top four the only drivers within a second of each other.
Magnussen ended the hour fifth ahead of Verstappen and team-mate Mick Schumacher, who impressed on his first visit to Suzuka.
Norris and McLaren overcame the power unit fears to finish eighth, ahead of Valtteri Bottas' Alfa Romeo and Red Bull's Sergio Perez.
Mercedes found an oil leak for George Russell in the morning and was forced into a change to another power unit in the pool, the Briton out of action for the first 40 minutes of the session.
Russell finished 18th quickest after complaining about tyre grip, with Lewis Hamilton only 13th in the other W13.
Despite two late laps from the Alfa Romeos, the final 10 minutes of FP1 were essentially washed out as heavy rain fell, with the weather forecast casting doubt over any running in FP2.
Cars returned to the track despite the conditions to complete practice race starts, but drama after the session saw Schumacher crash at the Dunlop Curve, leaving Haas with plenty of work to do for the rest of the day to make repairs.