JEREZ, Spain (AP) Sebastian Vettel posted the fastest lap for Ferrari on the first day of pre-season Formula One testing, while last year's championship runner-up Nico Rosberg clocked a massive 157 laps.
Rosberg lost out on the title to Mercedes teammate Lewis Hamilton in a gripping and often tense tussle last year that went down to the final race in Abu Dhabi.
''Today every lap counts, I can feel I'm rusty,'' Rosberg said. ''I'm definitely not as good now as I was on Sunday in Abu Dhabi. So I need to use these days to get back up to speed.''
The season opens on March 15 in Melbourne, and if Sunday's testing is anything to go by, Rosberg is in the mood to upstage Hamilton.
''I want to become an even better driver this year,'' Rosberg said. ''That's my aim and my personal challenge and it's a great challenge.''
The German driver was so keen to pile on the laps that he even had lunch sitting in his car.
''That was the goal, to do a lot of mileage. Of course I'm feeling pains here and there,'' he said. ''The good thing is that we haven't found any major problems yet. Reliability is very important for us. Try to find problems and get them sorted before the first race.''
Vettel's best lap of 1 minute, 22.62 seconds was .157 quicker than Swedish driver Marcus Ericsson and .486 better than Rosberg.
There are four days of testing. Hamilton drives on Monday and Wednesday.
Daniel Ricciardo was fourth fastest - .716 behind - and completed 35 laps in his Red Bull.
''The first impressions are good. We'll get a few more laps over the next few days, but so far everything seems pretty encouraging,'' the Australian driver said. ''I can feel a step forward. I won't go into too much detail but it is there.''
Ricciardo was the only driver to break Mercedes' stranglehold last season, winning three races and outdoing four-time F1 champion Vettel - who left Red Bull to join Ferrari.
''We knew (Mercedes) came out with a pretty awesome package last year and they probably just refined it and perfected it,'' Ricciardo said. ''I expect them again to be the ones to beat coming into Melbourne. But I'm not intimidated either. I think it's a reachable target for us.''
Ferrari is desperate to find a way back to the top in F1, and last year failed to win a race for the first time since 1993.
The team shakeup saw Fernando Alonso leaving to join McLaren, but it proved an unhappy day for the Spaniard as he managed only six laps.
The long session was interrupted twice, with the red flag coming out for the second time when Carlos Sainz Jr stalled his Toro Rosso.
Testing continues until Wednesday.