Australian Mark Webber topped the times yesterday in a rain-hit opening free practice session ahead of this weekend's potentially decisive Brazilian Grand Prix.

The 33-year-old Red Bull driver emerged on top after a disjointed crash-affected session that offered few clues to the outcome of this year's drivers' world championship.

Webber produced the best lap of the morning in a time of one minute and 12.463 seconds around the demanding old Interlagos circuit to finish narrowly ahead of local hero Rubens Barrichello of Brawn GP.

Webber's Red Bull team-mate Sebatian Vettel of Germany was third quickest, but championship leading Briton Jenson Button made his now almost-customary slow start to the action by finishing up seventh.

The McLaren Mercedes cars driven by Finn Heikki Kovalainen and outgoing world champion Briton Lewis Hamilton were fourth and fifth fastest.

On a cool morning, the session was plagued by rain which caused a 20-minute delay and then an accident involving Frenchman Romain Grosjean in his Renault, who slid off the track into a board that smashed to leave debris everywhere.

This created a further seven-minute interruption after which the cars returned tentatively to the circuit before another heavy rain shower signalled their return to the pits.u00a0- AFP