Toro Rosso driver Carlos Sainz set the pace in a drenched first practice session at the Japanese Grand Prix.
He was 0.504 seconds quicker than Red Bull driver Daniil Kvyat in a session marked by varying degrees of rain.
Both were on the lightly treaded intermediate tyre when they set their times. Mercedes' Nico Rosberg, third, was on the more heavily treaded wet.
Ferrari's Sebastian Vettel was fourth, also on the intermediate, ahead of Mercedes' Lewis Hamilton on full wet.
Only 12 drivers set lap times as many were cautious to go out because the weather is predicted to improve through the rest of the weekend.
The reason for their caution was underlined by a number of wobbles and off-track moments for several drivers.
Vettel held a lurid flick through the kink between the hairpin and Spoon Curve, which is flat out in the dry but not when it is more slippery in the wet.
Williams's Valtteri Bottas ran wide after hitting a puddle through the 130R curve, which is taken flat at 200mph in the dry, but held the car and was able to continue without incident.
And the Finn's team-mate Felipe Massa ran off the track through the difficult Degner Two corner, which often catches drivers out, but managed to keep the car away from the barriers.
The conditions meant it was not possible to see whether Ferrari will be able this weekend to continue the impressive form that saw Vettel dominate the race in Singapore last weekend.
The four-time champion was 0.442secs slower than Rosberg but the fact they were on different tyres meant it was not possible to compare their times.
Rosberg and Hamilton have said this weekend that they believe their unexpectedly poor performance in Singapore was specific to that race track.
However, Mercedes will be keen for some reassurance that that is the case.
And Hamilton also said he believed that Ferrari had improved their car over the last two races.
The team introduced an upgraded engine at the Italian Grand Prix three weeks ago - as did Mercedes - as well as aerodynamic revisions to their car, which worked particularly well in Singapore.
Rosberg was running on Friday morning with the new engine that he first tried in practice at Monza but which had to be removed from his car for the race because a problem was detected with it.
The unit appeared to run without problems here in Japan.
Red Bull's Daniel Ricciardo, who finished second in Singapore, said after that race that he did not expect his team to be as competitive in Japan, where the circuit layout means the lack of power of their Renault engine will be more of a handicap.
But the Australian did add that it would be "game on" if it was wet.
But there was not enough running to form any sense of a competitive order in the event these conditions were to continue for the rest of the weekend.
The highest number of laps done by any driver was 15 by Ferrari's Kimi Raikkonen, and Ricciardo was one of the eight drivers not to set a time.