Rosberg's new team mate Lewis Hamilton
will take over the car for the fourth and final day of this current Barcelona test session. After his most recent runs on Wednesday, the former world champion was also expressing positive thoughts about his new hardware for 2013.
"It's going in the right direction.," Hamilton said. "We're not focusing on anyone else; we're just looking at our programme to try to make steps forward with the degradation and set-up ... All the tyres are having degradation so looking after them is not that easy, but that's normal."
Hamilton was second-fastest on Wednesday to the man who replaced him at McLaren, Sergio Perez - although the Mexican was on softer tyres to Hamilton, making the comparison an inexact one at best.
Rosberg finished the Thursday session in fourth place behind Ferrari's Fernando Alonso, Sauber's Nico Hulkenberg
and Lotus driver Romain Grosjean, having put in 108 laps of the 2.892-mile, 16-turn circuit.