Rosberg and triple world champion Lewis Hamilton shared testing duties on Tuesday, with the Briton taking over in the afternoon for a race simulation. Together, they did 169 laps without a hitch
Nico Rosberg gave a hint of Mercedes' true speed on Tuesday, with the fastest time on the first day of Formula One's final pre-season test, and then said there was still room for improvement.
It was the first time in 2016 testing that the world champions had led the timesheets at the end of the day and Rosberg was not even using the quickest tyres to go faster than everyone else.
The German lapped the Circuit de Catalunya with a best time of one minute 23.022 seconds after using soft tyres for the first time this year.
He was two tenths of a second faster than Valtteri Bottas (1:23.229) who was using the ultra-soft tyres on his Mercedes-powered Williams.
"Today in particular we tried qualifying (simulations), to see where the one-lap limit is," Rosberg told the formula1.com website.
"It was riding on full attack on low fuel -- and that felt super...even if I have to say that there is still room for improvement."
McLaren's Fernando Alonso was third fastest, 1.7 seconds slower than Rosberg in a turn-up for the team after a nightmare 2015 season, while Ferrari's Kimi Raikkonen was fourth in 1:24.836.
However, the Finn suffered a gearbox problem in another worrying sign for Ferrari that reliability could be a weak spot this season.
"Obviously, it could be slightly better at this point but it's testing and the purpose of testing is to try things," said the 2007 champion.
"If something breaks and we have an issue, we can fix it, we still have time... So am I concerned? Not really."
Dutch teenager Max Verstappen did the most laps of the day, completing 144 in the new Toro Rosso whose 2016 livery was unveiled earlier.
Mercedes did far more mileage than their rivals in last week's first four-day test that focused more on reliability than outright performance.
Rosberg and triple world champion Lewis Hamilton shared testing duties on Tuesday, with the Briton taking over in the afternoon for a race simulation. Together, they did 169 laps without a hitch.
Sauber had their new car running for the first time, after bringing a 2015 car to the first test, and Brazilian Felipe Nasr did 103 laps.
The season starts in Australia on March 20.