Feb.28 (GMM) German racer Nico Rosberg has played down Williams' hopes of emerging as the third force in formula one this season.

The Grove based team has enjoyed a positive winter period with its new Toyota-powered FW30, but 22-year-old Rosberg doubts that the single seater is yet ready to take on BMW Sauber for the 'best of the rest' honours behind Ferrari and McLaren.

"I think fourth in the constructors' championship is the main target; and that would be a great achievement," he told the German magazine Auto Motor und Sport.

Rosberg insists that only "luck" will deliver him a maiden podium placing in 2008.

"For that to happen, at least one Ferrari, one McLaren and two BMWs need to fail. So it is going to need some luck," he said.

He does think Red Bull and Renault are firmly in Williams' sights, however, but confesses that he is "not ready to accept" that BMW can truly be challenged for third best.

Indeed, when asked by the German news agency DPA if Williams' true ambition for 2008 is third in the teams' title, Rosberg answered robustly: "No."

On the ksta.de website, he is quoted as saying he wants to be challenging for wins and the title in a few years, but that the big regulation changes for 2009 could give Williams a chance to get back in the hunt.

"Everything can be totally reshuffled then," he explained, "which is an opportunity not only for us, but for everyone."