Williams foresees a tough time ahead in the Italian Grand Prix at Monza next weekend following one of its most challenging outings of what has in general been an impressive season in 2009 in Belgium - where Sam Michael admitted the FW31 simply 'wasn't strong enough'.

Williams was far from alone in struggling around the high-speed, low-downforce Spa-Francorchamps circuit, with other anticipated front-running outfits like Brawn GP, McLaren-Mercedes and Red Bull Racing also finding it heavy going in the Ardennes - handing such as Force India and BMW-Sauber their chance to shine.

Way off the pace in Friday practice - in which Nico Rosberg has been something of a master in 2009 - a late night followed ahead of qualifying on the Saturday, but a raft of significant aerodynamic modifications to the team's Toyota-powered contender failed to yield any better than tenth place for the young German when it counted in Q3, as team-mate Kazuki Nakajima in the sister car dropped out as early as Q1 in a lowly 18th position.

On a comparatively heavy fuel load for the start of the race, Rosberg ran eighth for most of the duration, getting a sniff of a better result when the Brawn GP of Rubens Barrichello in seventh began to spew oil out of its rear end in the closing stages, but eventually having to settle for just a single marker at the close - the 24-year-old's eighth straight points-scoring result of the campaign, and tenth from twelve starts this year.

Nakajima, for his part, took the chequered flag only 13th - almost 20 seconds further in arrears - and the Grove-based concern's technical director Sam Michael acknowledged that it had not exactly been a vintage performance for the former multiple world champions.

"Our car wasn't strong enough," the softly-spoken Australian succinctly summarised. "Of course the gaps are very close, with our qualifying and race performance within 0.5 seconds of the fastest, but in the current competitive field that means being just inside the points.

"We really struggled during Friday practice. We changed a lot of the bodywork for Saturday and the car improved, but our low drag performance is not strong enough. Sector two [at Spa] is predominantly about downforce, and the FW31 has always been strong through sectors like that.

"Of course Nico did well to get the best out of it in qualifying and the race. We told him to be careful in case Barrichello's engine was about to let go [in the closing laps], but then we quickly noticed that his top speed and lap time was not that different so we assumed it was an oil tank problem rather than an engine issue.

"The two tyre compounds were quite close through Friday and Saturday. At Williams we favoured the 'Prime' tyre for the race, but in reviewing our choice post-race we perhaps could have been using the 'Option' tyre earlier as the car was most competitive on the softer rubber on Sunday.

"Kazuki struggled with the car during the weekend - and looking at how difficult it was for Nico to squeeze a point out of the race, it wasn't too surprising. He's still pushing hard and he should have a strong finish to the season. We don't expect to be very competitive at Monza, but we'll do our best to be respectable and look to return to strong form for the final four races of the season that are in the traditional lift-to-drag region."