Mercedes driver Lewis Hamilton headed Ferrari's Fernando Alonso in a tight first practice session at the Italian Grand Prix.
Hamilton was just 0.035 seconds quicker than Alonso, with the second Mercedes of Nico Rosberg third fastest ahead of Red Bull's Sebastian Vettel.
Just 0.188secs covered the top four as Lotus driver Kimi Raikkonen was fifth.
McLaren drivers Sergio Perez and Jenson Button were next, ahead of Red Bull's Mark Webber.
Force India's Paul Di Resta was 15th fastest, while Englishman James Calado took 17th in his first outing as the team's third driver.
Calado was 0.447secs behind his Scottish team-mate and will hand his car back to race driver Adrian Sutil for the second session, which starts at 1300 BST.
On the high-speed Monza circuit, which has relatively few corners, the field was closely packed - less than half a second separated Hamilton from seventh-placed Button. And the top 14 were covered by less than a second.
Heading into the weekend, the expectation was that the main contenders would be Hamilton, Alonso and Vettel, and first practice seemed to back that up.
Alonso, second in the championship, is desperate to cut Vettel's 46-point lead this weekend and it could be a good opportunity for him - Red Bull team principal Christian Horner said the team did not expect this to be one of their stronger circuits and told BBC Sport "a podium would be a good result for us this weekend".
He pointed out that Vettel was more than 10km/h slower than Hamilton on the straight - but it is worth remembering that the German was also relatively slow on the straight in 2011 when he took a dominant victory.
BBC F1 technical analyst Gary Anderson said: "Mercedes are looking in pretty good shape. The big question is: can they look after their tyres? This is, after all, only the second race with the new tyre spec that was introduced as a solution for the multiple failures at Silverstone.
"They have a decent level of downforce and a decent-looking front wing.
"Ferrari are in pretty good shape, but I think there is more to come from them.
"They have been trying different front wing assemblies and I've been told they have a different rear wing to try in the second session. They're probably just waiting for a bit more rubber to go down on the track so it is in a more representative condition.
"It's business as usual for Red Bull. Visually, they have the smallest rear wing of all the top teams but the car generates so much underbody downforce that they can do that and still keep good braking stability."