SILVERSTONE (AFP) - Australian Daniel Ricciardo topped the times in his Toro Rosso on Friday, as heavy rain reduced opening free practice for Sunday's Formula One British Grand Prix to a minimum of track action.

Standing water and the dangers of aqua-planing meant only 10 drivers ventured out to clock times with Ricciardo, a possible contender to replace compatriot Mark Webber at Red Bull next year, demonstrating courage and competence in the appalling conditions.

He was fastest with a best lap of 1min 54.279sec ahead of nearest rival German Nico Hulkenberg of Sauber by 0.794sec.

Venezuelan Pastor Maldonado was third in a Williams, 1.1sec adrift and local favourite 2008 winner Briton Lewis Hamilton of Mercedes fourth 1.2sec slower than Ricciardo.

Most of the leading drivers including defending triple world champion German Sebastian Vettel and his Red Bull team-mate Webber, Ferrari's two-time champion Spaniard Fernando Alonso and the 2009 champion Briton Jenson Button of McLaren chose not to enter the fray.

Webber announced on Thursday that he will leave Red Bull and Formula One at the end of the year to go sportscar racing with Porsche.

In the atrocious and treacherous conditions, it was remarkable that only one driver - Frenchman Charles Pic of Caterham - crashed. He who slid off the track at the exit from Club corner and smashed into the barriers, damaging his front wing.