Rosberg's time of 1:20.130 was the fastest time across any of the eight days of testing at the home of the Spanish Grand Prix.
The German driver then followed that time up with another 1:20.1 on a fresh set of soft tyres.
Fernando Alonso was second with a best time of 1:20.494, also set on soft tyres. Alonso also set times of 1:20.5 and 1:20.6 on another set of soft tyres.
Jenson Button was third, ahead of Nico Hulkenberg, whose race simulation came to a premature end when Sauber discovered a technical problem - thought to be a water leak - in his C32.
Having had several setbacks in his pre-season programme - including food poisoning which forced him to miss testing on Saturday - Kimi Raikkonen had to sit out most of the morning session as Lotus discovered a problem on his gearbox following his installation lap. He returned to the track with five minutes to go before lunch and set the fifth fastest time overall during afternoon running.
Aside from Lotus, most teams enjoyed a productive day as an absence of red flags enabled several drivers to pass the 100 lap barrier. The four fastest drivers, along with Force India's Paul di Resta and Caterham's Charles Pic all achieved the feat.
Williams again split driving duties between their drivers. Pastor Maldonado competed in the morning session before handing over to Valtteri Bottas. The duo finished seventh and ninth respectively, split by the Red Bull of reigning champion Sebastian Vettel.
Caterham and Marussia recorded their best times of the test as Pic set a time of 1:23.115 in his CT03. New Marussia recruit Jules Bianchi was narrowly behind with a 1:23.167, before handing over to fellow rookie Max Chilton for the afternoon session.
|6||Paul di Resta||Force India||1:21.664||109|
|8||Sebastian Vettel||Red Bull||1:22.514||96|
|12||Daniel Ricciardo||Toro Rosso||1:23.628||85|