Fernando Alonso's Ferrari overcame heavy rain which eventually triggered a premature end to the session to top the times at Tuesday's testing at Mugello.
Despite Alonso's win in Malaysia Ferrari have had a lowkey start by their high standards to the season, yet Alonso is still just 10 points off championship leader Sebastian Vettel in fifth.
With only one hour of action out on the circuit before lunch Alonso put in a time of 1min 22.444sec to edge the Red Bull of Mark Webber by over one second.
Several teams including McLaren and Lotus elected to field their reserve drivers in this three-day testing session which is the only one of the season held outside race weekends.
The sole team missing from Mugello, the first in-season tests since 2008, is HRT.
The next race is the Spanish Grand Prix on May 13.
1. Fernando Alonso (ESP/Ferrari) 1min 22.444sec, 2. Mark Webber (AUS/Red Bull) 1:23.648, 3. Jean-Eric Vergne (FRA/Toro Rosso) 1:23.891, 4. Jerome D'Ambrosio (BEL/Lotus) 1:24.048, 5. Nico Rosberg (GER/Mercedes) 1:24.100, 6. Kamui Kobayashi (JPN/Sauber) 1:24.736, 7. Oliver Turvey (GBR/McLaren) 1:25.303, 8. Jules Bianchi (FRA/Force India) 1:25.475, 9. Rodolfo Gonzalez (VEN/Caterham) 1:27.197, 10. Charles Pic (FRA/Marussia) 1:27.359, 11. Valtteri Bottas (FIN/Williams) 1:29.179, 12. Oliver Turvey (GBR/McLaren) 1:50.898