Overnight rain created damp track conditions which led to a largely uneventful session as riders were reluctant to explore the Aragon circuit.
It was left to newcomer Luca Scassa and Claudio Corti to provide any entertainment in the first 30 minutes as the pair made good use of the session and took turns at the top of the timesheets.
Bradley Smith was the first to join the duo and posted the first meaningful lap time with a 1m55.770s, which acted as a catalyst for the other riders to join in and led to a brief flurry of activity towards the end.
Half of the field headed out onto the track in the final five minutes and Rossi went fastest of all with his 1m50.464s, ahead of Alvaro Bautista and Nicky Hayden.
Aleix Espargaro was 2.3s adrift of the ultimate pace, while erstwhile leader Smith dropped to fifth, ahead of Hiroshi Aoyama and Andrea Iannone.
Those not posting lap times in the session included current championship leader Marc Marquez and Honda team-mate Dani Pedrosa, as well as title rival Jorge Lorenzo and Tech 3's Cal Crutchlow.