Lewis Hamilton topped the times for Mercedes in Friday morning's opening free practice session ahead of this weekend's Italian Grand Prix.
The 28-year-old Briton, running a revised low-downforce package following the team's disappointing outing at the Belgian Grand Prix two weeks ago, outpaced nearest rival Fernando Alonso of Ferrari by just 0.035 seconds when he clocked a late flying lap in perfect conditions.
Under a blue sky and warm sunshine, Hamilton clocked a best lap of one minute and 25.565 seconds with five minutes remaining.
Alonso, seeking to prove to the tifosi that he can engineer a Ferrari recovery in a dismal season by their standards, did his utmost to regain the leading position, but a slight mistake at the exit of the Parabolica left him frustrated.
Hamilton's Mercedes team-mate Nico Rosberg of Germany was third fastest ahead of compatriot and defending triple world champion Sebastian Vettel of Red Bull, Finn Kimi Raikkonen of Lotus and the two McLarens of Mexican Sergio Perez and Briton Jenson Button.
Australian Mark Webber was eighth fastest in the second Red Bull, but only half a second off the pace, ahead of Venezuelan Pastor Maldonado of Williams and Frenchman Jean-Eric Vergne of Toro Rosso.
Webber's fellow-Australian Daniel Ricciardo, who will replace him at Red Bull next year, was down in 13th in his Toro Rosso, one place ahead of Brazilian Felipe Massa in the second Ferrari.
The top 14 were all within nine-tenths of a second of Hamilton's time.
Vettel leads the championship with 197 points ahead of Alonso on 151 and Hamilton on 139.