Mercedes endured a nightmare start to pre-season in Jerez two weeks ago as Rosberg completed just 14 laps on day one after the new W04 sustained an electrical fault that sparked a brief fire.
Twenty-four hours later and Lewis Hamilton, on his debut in the car after joining Mercedes following six years in F1 with McLaren, was pitched into a tyre barrier by a rear brake failure after 15 laps.
The team managed to atone over the following two days as Rosberg and Hamilton chalked up nearly 300 laps between them to prove the car is reliable.
Although Rosberg suffered gearbox issues early on on the first day of the second four-day test at Barcelona's Circuit de Catalunya, he ultimately finished top of the timesheet.
With a lap of one minute 22.616secs, 0.9secs slower than Hamilton's pole for last year's Spanish Grand Prix at the same track prior to him being dropped to the back of the grid for a fuel irregularity, Rosberg was happy enough after what he described as "a mixed day".
"It's always nice to be quickest at the end of the day," said Rosberg.
"In general, I feel happy and comfortable in the car, and it feels better than the last time we were here at this track.
"The car feels decent out there. It has a much, much better base. You can feel its centre of gravity is lower, the aero is better, the platform is better.
"It feels a lot stronger. It's a pleasure to drive because it's a lot faster than last year."
Rosberg is, however, concerned by the reliability of the car given the issues encountered so far, completing just 54 laps.
"That's a bit of a pity. We have to work on that," added Rosberg.
"We have a few too many reliability issues at the moment. Other than that, it's definitely going in the right direction."
Behind Rosberg, Lotus' Kimi Raikkonen finished just seven thousandths of a second adrift after the Finn, like the German, also finished the day on a set of medium Pirelli tyres.
On his return to the cockpit after skipping the first test to hone his fitness, Fernando Alonso clocked up a day-high 110 laps in his Ferrari, with his best time a third of a second behind Rosberg, and narrowly ahead of Red Bull's triple world champion Sebastian Vettel.
Alonso described the car as nothing more than "okay", albeit a vast improvement on his first day in the cockpit a year ago.
"There was nothing really surprising, so this is at least something good, unlike last year when we drove for the first time and we were a little surprised in a negative way," said Alonso.
"But we have a lot of work to do with the tyres, aerodynamics, some new components and experimental things we have in the car.
"We need to keep doing a lot of mileage. In Jerez we had mechanical problems, so we need to put some good laps in this test to prove the car is strong enough to race."
As for Vettel, he issued an ominous warning as he said: "I'm quite happy with the car and with the balance, quite a bit happier than where we started last year."
As the last team to unveil their car on Tuesday, the Williams FW35 was a competitive fifth quickest at the hands of Pastor Maldonado whose best time of his 86 laps was a 1:23.733.
However, motor sport's governing body, the FIA, have voiced their opinion the car, and that of Caterham, is currently using an illegal exhaust which will have to be changed before the season-opening race in Melbourne on March 17.
Behind the Venezuelan were Toro Rosso's Daniel Ricciardo and Sergio Perez for McLaren, the Mexican 1.5secs adrift.
Force India's Paul di Resta finished in close attendance to Perez, with the remaining three drivers in Sauber's Esteban Gutierrez, Max Chilton for Marussia and Caterham's Charles Pic all considerably off the pace, with the latter also suffering a gearbox issue.