Toro Rosso's Carlos Sainz Jr was quickest in first practice for the Japanese Grand Prix as persistent rain kept Formula 1 running to a minimum at Suzuka.
The Spaniard clocked a 1m49.434s, half a second clear of Red Bull's Daniil Kvyat with Mercedes' Nico Rosberg third.
Singapore GP runner-up Daniel Ricciardo was one of eight who chose not to set a time, with conditions getting worse and several drivers complaining about aquaplaning in the closing stages.
There was very little running in the first 50 minutes, with most simply doing an installation lap before heading back to the pits.
Toro Rosso's Max Verstappen set the first timed lap with a 1m51.741s on the full wets but as the rain eased, Williams drivers Valtteri Bottas and Felipe Massa were the first to switch to the intermediates.
With 12 minutes to go, reigning world champion Lewis Hamilton set his first timed lap, with the Briton improving to finish fifth, two tenths adrift of Ferrari's Sebastian Vettel.
Hamilton's team-mate Rosberg was running his Monza engine, which he was unable to use in the Italian race after the team discovered a coolant leak that damaged the revised unit, to test it having fired it back up in Singapore.
Mercedes said the German would switch back to his fourth power unit - which he ran in Singapore - for the rest of the weekend.
Verstappen was sixth, ahead of Ferrari's Kimi Raikkonen, Massa, Sauber's Marcus Ericsson and Bottas, who had a big moment at 130R when he hit a patch of water.
Felipe Nasr in the other Sauber and McLaren's Jenson Button completed the list of drivers to set a time.
Conditions are expected to improve for second practice, which takes place later on Friday.
PRACTICE ONE TIMES:
|1||Carlos Sainz Jr.||Toro Rosso/Renault||1m49.434s||-||10|
|2||Daniil Kvyat||Red Bull/Renault||1m49.938s||0.504s||7|
|6||Max Verstappen||Toro Rosso/Renault||1m50.940s||1.506s||10|
|13||Daniel Ricciardo||Red Bull/Renault||-||-||1|
|14||Nico Hulkenberg||Force India/Mercedes||-||-||4|
|15||Sergio Perez||Force India/Mercedes||-||-||3|