Jenson Button admits McLaren has been surprised by how quick Williams looks in race trim at the Italian Grand Prix.

The two Williams cars will start from third and fourth at Monza, and Friday's practice sessions saw the team deliver strong race pace on the longer runs. With the two McLarens starting directly behind Valtteri Bottas and Felipe Massa on the grid, Button said he expects the main focus to be trying to hold off Fernando Alonso and the two Red Bulls behind.

"I think we'd always want to move forward but realistically I think we were slightly surprised at how quick the Williams was compared to us in terms of lap time," Button said. "I think we expected to be fighting with the Ferraris and Red Bulls and being fifth and sixth we should be very happy with that because we thought we'd be in a scrap with those guys but we're actually just in front so we've done a good job there."

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And Button even went as far as to pick the Williams out as the fastest car on high fuel.

"To challenge the Williams tomorrow is very, very difficult looking at the pace of the car on a long run; they actually looked the quickest car yesterday. For us it's more the challenge with the Ferraris and the Red Bulls because they also seemed pretty competitive yesterday.

"We have improved the car since yesterday. I think we'll fine-tune it a little bit before tomorrow and hopefully our pace will be good enough to fight for fifth. Then obviously if the guys in front slip up then we can take the position."