Valtteri Bottas says he is not satisfied with fourth place in the Italian Grand Prix because he had felt Williams could challenge Mercedes at Monza.

Having qualifying in third place, Bottas had a very bad start and slipped back to 11th by the end of the first lap. An impressive recovery drive saw him eventually finish fourth behind team-mate Felipe Massa, and despite the strong result Bottas said he was disappointed not to get the chance to try and attack Mercedes.

"It's a pity because today I came in before the race with the mentality that this is probably the best chance of the season to fight for the win," Bottas said. "So fourth position is not really that satisfying but we need to look at the positives because in the end we got good points for the team. Now we are ahead of Ferrari which I think if you compare to last year is a really good achievement."

Having had such a poor start, Bottas said he feels Williams didn't quite get its settings right for the launch on the grid.

"We had a good look in the debrief and we're still going to analyse it more but compared to the grip I had in the tyres I think we were too aggressive today with the clutch. The start on the formation lap - when we always do the final checks - was a bit compromised because Lewis had a slow getaway, so that maybe hurt a little bit. But definitely the tyres were not in the optimum window.

"Maybe for that condition we were a bit too aggressive because I had a poor start but the same thing continued after Turn 2; just wheelspinning up to fifth gear. After the poor start, positions I was in in the chicane sometimes I was a bit unlucky and I couldn't accelerate at the maximum rate, so that really compromised the race for me."

At one stage Bottas went off at Turn 1 fighting Kevin Magnussen - an incident which saw Magnussen penalised - and he said he could understand why the stewards had handed out a penalty even if it was a tight call.

"It was on the limit, yeah. It's difficult to say if it was worth a penalty. Of course for me there was nowhere else to go, I had to cut the chicane and I didn't have the chance to overtake even though we were really side-by-side. It's a tough one, in the end for me it didn't change anything even though he got a penalty because I had to still overtake him, so for me it didn't really matter."