Red Bull driver Sebastian Vettel edged out McLaren's Lewis Hamilton to set the pace in Italian Grand Prix practice.
Hamilton was held up in traffic and finished 0.036 seconds behind the championship leader.
But Hamilton set the day's fastest time in the morning session, nearly a second ahead of Button in second.
Michael Schumacher was a surprise third for Mercedes from the Ferraris of Felipe Massa and Fernando Alonso, with Button seventh in the afternoon.
Vettel leads the championship from team-mate Mark Webber by 92 points after winning the last race in Spa but the German insisted he would not be the favourite this weekend.
With just a single high-speed corner at Parabolica and four high-speed straights, Monza's low-downforce circuit should not play to Red Bull's strengths.
But Red Bull appeared to have heavily updated their car for the unique challenges of this weekend and Vettel unleashed a lap of one minute 24.010 seconds to see off his rivals.
"This is not what anybody expected, this is a surprising performance," said BBC 5 live analyst Maurice Hamilton. "Spa was not expected to be a good circuit for Red Bull either but they finished one-two."
Vettel said after the session that although he was pleased with the way his car felt, he thought McLaren looked like the strongest team.
Hamilton had arrived in Italy with his own agena as he was determined to deflect any lingering criticism from an eventful Belgium Grand Prix.
The 2008 world champion was reprimanded for a collision during qualifying with Williams driver Pastor Maldonado and then crashed out of the race after he collided with Kamui Kobayashi's Sauber in an early battle for fourth.
Hamilton set the pace in the morning session and chased Vettel hard in the afternoon, saying afterwards that he "wasn't worried about the Red Bulls."
"Both me and Jenson [Button] are very happy with the car," he said.
"We are in a stronger position than the Ferraris on heavy fuel, the Red Bulls were quick at the end but I'm pretty sure they were not on the same fuel load as us.
"I feel optimistic. I think we can challenge the Red Bulls.
"It will be a very hard race in these conditions. We are strong and in a good position but to beat the Red Bulls will be tough. It is not impossible, though."
The pair were on the track at the same time when they set their fastest laps but Hamilton had to lift off the throttle as he tried to go even faster to avoid colliding with Jamie Alguersuari's Toro Rosso through Parabolica.
Mercedes demonstrated improved straight-line speed in Spa and Schumacher's time - just 0.337 secs behind Vettel's benchmark - hinted they could perform well in qualifying at the very least despite Nico Rosberg's lowly 21st place.
The team had also tried a different tactic in the morning, running on high fuel in an attempt to improve their competitiveness over the race distance.
As track temperatures climbed to 40C in the late summer sunshine, Ferrari also eked out some extra pace after finding themselves three seconds slower than Hamilton's leading McLaren in the morning session.
Alonso finished the day 0.423 secs off the pace and just behind team-mate Massa but the team conceded they were looking for more at their home race.
The demands of Monza's Parabolica, caught out several drivers, including five-time Italian GP winner Schumacher, who ran onto the gravel in the morning.
But Sebastien Buemi day's ended at the famous corner when he ran onto the grass, clipped the tyre barrier and beached his Toro Rosso into the gravel in the afternoon.
The Swiss driver summed up the incident on the pit-to-car radio: "Nothing happened to me, but the car is broken."