|© AP Photo/Tim Hales
Japanese driver Kamui Kobayashi was fastest in the rain-soaked first practice session at the Belgian Grand Prix on Friday, with championship leader Fernando Alonso preferring to stay in the Ferrari garage.
Defending champion Sebastian Vettel waited until near the end of the session before posting the ninth quickest time in the hard rain, but he was holding his Red Bull back on the slippery circuit.
Kobayashi clocked 2 minutes, 11.389 seconds for Sauber, with Venezuelan rival Pastor Maldonado second quickest in 2:11.941 for Williams, followed by Daniel Ricciardo of Australia in his Toro Rosso.
Felipe Massa tried to come out in the closing stages but decided against it as smoke started to spew from the rear of his Ferrari.
More rain was expected for Friday afternoon's second practice run.
Besides a soaking track, drivers had to contend with dark clouds and low visibility, while some fans huddled under umbrellas and others sought shelter where they could.
Full wet tyres were necessary on the treacherous 7.004-kilometer (4.352-mile) track, which is the longest on the calendar and widely considered the most demanding along with Monaco.
Kobayashi was the only driver to do a timed lap for the first 45 minutes as other drivers, having done their install runs, chatted to engineers as they waited for conditions to improve.
With puddles of water forming at La Source, the first corner hairpin on the circuit, and other corners such as Eau Rouge and Blanchimont -- among the most difficult in F1 -- it was too dangerous to take needless risks in practice.
Ricciardo and French teammate Jean-Eric Vergne were the first to venture back out as the rain briefly stopped, followed by German driver Timo Glock in his Marussia and then a steady trickle of others.
But the rain did not hold off for long and Mexican driver Sergio Perez had a skid coming out of a corner and went across the track before getting his Sauber back under control.
Several drivers, including Alonso, lined up to start before changing their minds and going back inside.
Mark Webber, second in the overall standings behind Alonso and two points ahead of third-place Vettel, was fifth quickest behind Vergne.
McLaren drivers Lewis Hamilton and Jenson Button were 15th and 16th, respectively, but neither pushed hard.