The 2009 world champion will start from sixth on the grid in tomorrow's race at Monza alongside teammate Kevin Magnussen on the third row, after the rookie qualified fifth.
The Briton told Sky Sports News: "We've looked pretty good all weekend, and we gradually tweaked the set-up and balance through every session. On my final flying lap in Q3, I didn't get a clean middle sector, I lost a little bit of time through the Lesmos, but we probably qualified better than expected.
"I'm a little surprised at how quick the Williams cars have been, we didn't expect them to be that far in front of us, but we're the third-best team on single-lap pace, which is nice.
"I think things will be a bit more competitive in the race, our Friday long-runs proved somewhat tricky for us, so we'll be aiming for a better race tomorrow. It'll be hotter on Sunday, too, which could potentially help us, so we'll see what happens in the race."
Button has finished on the podium just once so far this season.