Sauber's Kamui Kobayashi set the fastest time as practice at the Belgian Grand Prix was affected by heavy rain.
The first track action for five weeks, following Formula 1's summer break, was seriously hampered by the weather.
There was limited running in the first session and the second was washed out - only 18 drivers ventured out and none did a flying lap.
Kobayashi ended the first session 0.552 seconds quicker than Williams driver Pastor Maldonado.
Ferrari's Fernando Alonso, the world championship leader, was among a handful of drivers not to do a flying lap in either session. That list also included both Lotus drivers, Kimi Raikkonen and Romain Grosjean.
McLaren's Lewis Hamilton did only one quick lap in the morning, which was enough to put him 15th, one place ahead of team-mate Jenson Button.
"It's been incredibly wet," Hamilton said. "Lots of rivers on the track, with no real benefit bombing around, except trying to find out if the updates were working.
"Our rear wing, which is especially for here and Monza, we know we had some problems at the beginning. So we just had to make some modifications to ensure it was working perfectly, and it is, but we won't see until tomorrow how good the upgrades are working," he added.
"That's the challenge - everyone is in the same boat. So it's going to be very interesting trying to find the correct set-up in the short space of time we'll have tomorrow.
"Hopefully our simulator has helped us so we are best prepared, with tomorrow the first real test of whether the set up is there or thereabouts."
Of the other leading figures, Red Bull's Mark Webber, second in the championship, was fifth fastest and team-mate Sebastian Vettel ninth.
Webber said: "It wasn't too bad in P1, the conditions were a lot better than in this [later] session. Extremely wet, lot of standing water, this session has been a bit of a write-off. [It's been a] difficult day for everyone and we're looking forward to coming back tomorrow."
Vettel added: "It's not the best weather for us because there is too much water. We would love to go out but we can't. It's a shame for the people round the track, they have been quite brave today. Hopefully they will come back tomorrow and we will have some sunshine. That's the forecast."
Mercedes driver Michael Schumacher, celebrating his 300th grand prix, managed to control his car after a lurid slide in the famous Eau Rouge corner in the first session.
And Ferrari's Felipe Massa pulled off in a big cloud of smoke at the end of the morning with an engine failure.
The rain continued over the lunch break, and with no improvement in the conditions the teams and drivers all decided it was too dangerous to run.
Caterham's Heikki Kovalainen was one of the drivers to do an exploratory lap in the afternoon and he reported rivers running across the track at several corners and aquaplaning.
Force India driver Nico Hulkenberg said: "There's so much water out there, going into and out of Eau Rouge. It's pretty dangerous out there."
BBC F1 technical analyst Gary Anderson, the former technical director of the Jordan, Stewart and Jaguar teams, said it made no sense to run in such weather because of the risk of crashing and causing damage that could run to hundreds of thousands of pounds.
It had rained overnight and all morning and a heavy downpour 10 minutes before the start of the session ensured the track was well and truly soaked.
There was little running in the first hour of the first session and those drivers who did venture out found the track too wet for meaningful work.
Hamilton had a nasty moment when his McLaren aquaplaned approaching the fast Blanchimont corner on his first lap and he abandoned any attempt to run, preferring to wait until conditions improved.
The conditions meant the teams were unable to make meaningful analysis of the large number of upgrades they have brought for their cars, and many remain a secret for now.
Ferrari say they have a number of new parts as they seek to improve the pace of their car and help Alonso hold on to his championship lead.
One new development that was obvious was at McLaren, who have new, small wings on the bodywork beside the driver.
Technical director Paddy Lowe said the team were pleased with the data they had logged but that the conditions meant it was not possible to form a full picture of the affect of the changes.
Lotus had their 'double DRS' straight-line speed boosting system on both cars for the first time after trialling it with Raikkonen in Germany and Hungary in July.
Anderson said he believed the team's intention was to run it throughout the weekend.
The device feeds air from intakes behind the driver's head up a central pillar on the rear wing to 'stall' it, reducing drag along the straights.
Like a different system Mercedes have been running all season, it is in addition to the standard DRS system run by all the cars. It is believed to operate separately from the DRS and to be usable at all times, while DRS is restricted in the race.
Tail-enders Marussia also have a 'double DRS' system they hope to try.