Fernando Alonso was quickest in the second practice session for Sunday's Italian Grand Prix, ahead of Lewis Hamilton, as McLaren overtook Ferrari.
Alonso suffered an engine misfire in the morning session
In the first session, Kimi Raikkonen pipped Ferrari team-mate Felipe Massa.
But double world champion Alonso picked up the pace in the afternoon as he looks to close the five-point gap to championship leader Hamilton.
Finn Raikkonen dropped down to eighth fastest around the Monza circuit, while Massa spun on his way to sixth.
Raikkonen said: "The first session went well but in the second I lost a lot of time with a hydraulic problem and only managed to do one run of laps on new tyres."
Alonso warned reliability would be crucial in the run-up to the end of the season.
"Any mechanical problems that any of the four (title chasing) drivers have in the remaining races will probably end their chance to win the title," he said.
Britain's Hamilton leads the drivers' championship race ahead of Spaniard Alonso, with Brazil's Massa 15 points back, one ahead of Raikkonen.
"I still feel quite relaxed," said Hamilton, 22. "Everyone else should feel the pressure as they have more work to do than me."
Giancarlo Fisichella, in a Renault, gave home fans something to cheer by clocking the third best time ahead of Poland's Robert Kubica in a BMW Sauber and German Nico Rosberg in a Williams.
The afternoon session was halted for nine minutes to allow Briton Anthony Davidson's stricken Super Aguri to be moved from the track.
Second practice session times for the Italian Grand Prix
1. Fernando Alonso (Spain) McLaren 1 min 22.386 secs
2. Lewis Hamilton (Britain) McLaren 1:23.209
3. Giancarlo Fisichella (Italy) Renault 1:23.584
4. Robert Kubica (Poland) BMW Sauber 1:23.599
5. Nico Rosberg (Germany) Williams - Toyota 1:23.679
6. Felipe Massa (Brazil) Ferrari 1:23.722
7. Nick Heidfeld (Germany) BMW Sauber 1:23.821
8. Kimi Raikkonen (Finland) Ferrari 1:23.833
9. Heikki Kovalainen (Finland) Renault 1:23.848
10. Alexander Wurz (Austria) Williams - Toyota 1:23.881
First practice session times for the Italian Grand Prix
1. Kimi Raikkonen (Finland) Ferrari 1 min 22.446 secs
2. Felipe Massa (Brazil) Ferrari 1:22.590
3. Lewis Hamilton (Britain) McLaren 1:22.618
4. Fernando Alonso (Spain) McLaren 1:22.840
5. Nico Rosberg (Germany) Williams - Toyota 1:23.472
6. Jenson Button (Britain) Honda 1:23.668
7. Giancarlo Fisichella (Italy) Renault 1:23.671
8. Robert Kubica (Poland) BMW Sauber 1:23.703
9. Nick Heidfeld (Germany) BMW Sauber 1:23.886
10. Jarno Trulli (Italy) Toyota 1:23.965