Marco Simoncelli managed the quickest time in practice for Sunday's Italian MotoGP at Mugello.
The Gresini Honda rider continued his recent form to top the timesheets in the morning session at his home track.
Championship leader Casey Stoner was second ahead of another Italian, Andrea Dovizioso, with Jorge Lorenzo fourth.
British rider Cal Crutchlow was down in 11th before afternoon rain saw only a handful of riders venture out onto the circuit for the second session.
Dani Pedrosa marked his return from a broken collarbone with the seventh-best time.
Simoncelli has qualified in the top two for the last five races and produced a superb final lap of the opening session as he finished the morning marginally ahead of Stoner.
In contrast, nine-time world champion Valentino Rossi endured a frustrating day in his native Italy as he suffered electrical problems with his Ducati and could only post the 13th best time.
Only Dovizioso managed a decent time in the afternoon session as just 12 of the 16 riders were credited with a lap.
Some only used wet tyres while Simoncelli, Stoner, Lorenzo and Pedrosa stayed in the garage throughout.
Crutchlow said: "Without doubt, this is the most difficult track I've had to learn so far this year, but I think 11th position is still a good result.
"The track is so technical and there is a lot of undulation, a lot of blind corner entries and you can't see the exit of the fast chicanes.
"I'm just upset because I've lost another session because of the rain and that is the last thing I need when I'm trying to learn the track and set some competitive times."
Italian MotoGP first practice times:
1. M Simoncelli (Ita) Gresini Honda 1'48.987
2. C Stoner (Aus) Repsol Honda 1'49.027
3. A Dovizioso (Ita) Repsol Honda 1'49.088
4. J Lorenzo (Spn) Yamaha 1'49.251
5. B Spies (USA) Yamaha 1'49.364
6. C Edwards (USA) Tech 3 Yamaha 1'49.810
7. D Pedrosa (Spn) Repsol Honda 1'50.104
8. N Hayden (USA) Ducati 1'50.203
9. H Aoyama (Jpn) Gresini Honda 1'50.629
10. A Bautista (Spn) Suzuki 1'50.658
11. C Crutchlow (GB) Tech 3 Yamaha 1'50.683
12. R De Puniet (Fr) Pramac Ducati 1'50.729
13. V Rossi (Ita) Ducati 1'50.729
14. H Barbera (Spn) Aspar Ducati 1'51.153
15. T Elias (Spn) LCR Honda 1'51.868
16. K Abraham (Cze) Cardion Ducati 1'51.876