MELBOURNE, March 15 (Reuters) - BMW Sauber drivers Robert Kubica and Nick Heidfeld lapped quickest in Saturday's final free practice for the season-opening Australian Formula One Grand Prix.

Poland's Kubica lapped the Albert Park circuit in one minute, 25.613 seconds, with Germany's Heidfeld 0.337 slower.

Spain's double world champion Fernando Alonso, who was dethroned last year by Finland's Kimi Raikkonen, was third for Renault in a low-key session for favourites Ferrari and McLaren.

Raikkonen, last year's winner in Melbourne limbering up for his first race as champion, was 14th on the timesheets after an off-track excursion while McLaren's 2007 overall runner-up Lewis Hamilton was 12th.

Germany's Nico Rosberg, who was sidelined for much of Friday due to a gearbox problem, completed 17 laps on a warm morning and was fourth fastest ahead of the Red Bulls of Briton David Coulthard and Australian Mark Webber.

Qualifying for Sunday's race was due to start at 0300 GMT.

(Reporting by Alan Baldwin, editing by Peter Rutherford)