French newcomer Charles Pic had the fastest time in Friday's second practice session at the Belgian Grand Prix, although none of the drivers posted a competitive lap as rain drenched the Spa circuit.
The 22-year-old Marussia driver had the best time of 2 minutes, 49.354 seconds but six drivers - including Red Bull's Mark Webber - preferred not to go out in treacherous conditions.
"The conditions have proved frustrating for everyone as there has been very little let-up in the rain," Pic said. "This is not unusual for Spa, though."
Webber felt the conditions were close to unmanageable.
"You can't race when there's so much standing water on track and you probably wouldn't qualify in this," said Webber, who is second overall behind Ferrari's Fernando Alonso in the title race. "So in the end we just put the car to bed tonight and come back tomorrow."
Australian Daniel Ricciardo was second fastest, followed by Alonso.
In equally wet conditions earlier Friday, Japanese driver Kamui Kobayashi was fastest in the first practice in 2:11.389 seconds, with championship leader Fernando Alonso again preferring to stay in the Ferrari garage and defending champion Sebastian Vettel ninth fastest.
"We'd love to go out on track more, but you can't," Vettel said. "You can't race in these conditions, as the tires can't get rid of the water."
Weather conditions are expected to improve for Saturday's third practice run and then qualifying.
With free time on their hands, Vettel strolled around the paddock, while teammate Webber chatted to Red Bull team principal Christian Horner, and Brazilian driver Bruno Senna moved up and down on the spot as he tried to stay warm.
Besides a soaking track, drivers had to contend with dark clouds and low visibility, while some fans huddled under umbrellas.
"It's a shame for fans around the track. It's quite windy and probably not so nice out on the grandstands," Vettel said. "Hopefully they can come back tomorrow with dry clothes and we'll have some sunshine."
It was too dangerous to take needless risks with puddles of water forming at La Source, the first corner hairpin on the circuit, and at other corners such as Eau Rouge and Blanchimont, among the most difficult in F1.
After a wait of nearly 50 minutes, Mercedes driver Nico Rosberg was the first to start the second practice, followed shortly after by Finland's Heikki Kovalainen, and veteran Michael Schumacher.
"Although it was raining hard, we learned a little about the behavior of the car today," Rosberg said.
The 43-year-old Schumacher, who is set to compete in his 300th GP on Sunday, said it was hard to keep the Mercedes on the track.
"There was a lot of aquaplaning," he said. "Unfortunately the weather today did not allow us to do much more than just basic work as there was too much rain coming down in both sessions."
Kovalainen soon span his Caterham car around as the treacherous 7.004-kilometer (4.352-mile) track became increasingly slippery.
Earlier, Felipe Massa tried to come out in the closing stages of the first practice but decided against it as smoke started to spew from the rear of his Ferrari, with his team later confirming that it was engine trouble.