"We know how strong McLaren will be here. They are probably favourites for pole.
"We know the potential of the Red Bull car - taking nearly all pole positions this year.
"It will be difficult but we are there and we will fight."
The pace of Alonso and Massa will be a boost for Ferrari, who have admitted they need a strong result at their home race to revive the Spaniard's title chances.
He has fallen 41 points behind Hamilton following a non-finish at the Belgian Grand Prix, with only 150 points still available in the remaining six races.
At their home race, the pressure is intense on Ferrari, who two days ago escaped further punishment from governing body the FIA over their apparent use of team orders at the German Grand Prix in July.
Massa managed to rescue a lurid off-track moment at the fast Parabolica corner after setting his best time.
The Brazilian ran off the track at the exit of the bend, where cars are doing more than 150mph, and skidded through the gravel trap.
But he avoided hitting the wall and he steered across the track and back into the pits.
Highlights - Italian GP first practice
Button set the pace in the morning session as he and Hamilton traded fastest times with championship rivals Red Bull.
The world champion, Hamilton and Red Vettel all took turns at the top.
Button was just 0.097 seconds ahead of Vettel, with Hamilton third.
Renault's Robert Kubica and Mercedes' Nico Rosberg were next, ahead of Webber, Force India's Vitantonio Liuzzi and Alonso.
In the afternoon, Williams's Rubens Barrichello was seventh ahead of Kubica, the second Williams of Nico Hulkenberg and Rosberg.
Ferrari were not competitive in the first session, when Alonso was 0.850secs off Button's pace. Ferrari said on their Twitter feed that both drivers were "not too happy with the grip".
The teams spent their time on Friday analysing the set-ups they will need at the unique Monza track, the fastest on the F1 calendar.
The teams run their cars here with less aerodynamic downforce than at any other circuit and finding the right balance between straight line speed and cornering is key.
It is made more complicated this year by the presence on many cars of the F-duct aerodynamic device, pioneered by McLaren but since adopted by several other teams.
The device reduces the car's drag on the straights, allowing them either to have fastest straight-line speed for the same cornering pace, or to run more downforce in the corners without a penalty down the straights.
But the device has its own inherent drag, and the balance between running it and not running it is finer at Monza than anywhere else.
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