MERC'S 2016 CONTENDER: Reigning world champions Mercedes unveiled its new F1 car. Image: Supplied
Barcelona - Formula 1 returns Monday for pre-season testing at the Circuit de Catalunya in Barcelona with teams still trying to catch Mercedes, the sport's dominant force in the last two seasons.
Mercedes hasn't been seriously challenged since major rule changes were introduced, leaving drivers struggling to keep pace with defending champion Lewis Hamilton and Nico Rosberg.
The four days of testing should provide a clue into whether teams have made any gains in their pursuit of Mercedes or if they will be fighting for second again. The second and final test sessions are scheduled for March 1-4, also in Barcelona.
Merc's new car
Mercedes was able to get an early feel for its new car during a promotional event a few days ago, and based on Rosberg's first impressions it's not good news for the rest of the grid.
"Here we go, what an awesome feeling, first lap with the new Silver Arrow," Rosberg said in a video released by the team. "Flat-out down the stretch, eighth gear, it feels really quick already."
The car was unveiled on Sunday on the team's website.
Here are other things to watch out for in the first pre-season test:
Ferrari was the team that got closest to Mercedes last year, with four-time champion Sebastian Vettel the only driver to win races in addition to the Mercedes' duo. Vettel finished third in the standings and Ferrari was second in the constructors' championship.
Vettel said: "Naturally our goals are growing. We all want more and hopefully this car will help us achieve it. Very excited to get on track and see how it feels."
The German driver said he could feel around the factory that the team is extra motivated to raise the bar and contend for the title.
Vettel said: "Everybody is fired up and we want to win, so the target is clear, the expectation is there. We will see how close we are and how much there is to do during the season."
A step forward for Williams
Williams was third in the constructors' championship the last two seasons and is working to take a step forward in 2016. It only had four podium finishes in 2015.
Deputy team principal Claire Williams said the team is expected to be challenging at the front this season.
Williams said: "There will be a number of strong teams who have us in their sights. We started design work on the car earlier than normal and hopefully this extra development time will stand us in good stead."
Better fortunes for McLaren, Red Bull
McLaren and Red Bull are eager to get on track again after a disappointing season in 2015. Red Bull couldn't manage better than a fourth-place finish in the constructors' championship, while McLaren was second-to-last after switching to Honda engines.
McLaren, which unveiled its new car on Sunday, is expected to show improvement compared to last year, although it's unlikely to be enough to allow the team to start contending for podiums or victories, which could mean another frustrating season for two-time world champion Fernando Alonso and one-time champion Jenson Button.
Red Bull has already warned that the team likely won't peak until the second half of the season.
Red Bull chief engineer Paul Monoghan said: "We put in a phenomenal effort through last year. Many skilled people put in a lot of dedicated hours, and we are judged relative to the front-running car. If that's us, fantastic, if it's not, we've got a lot of work to do."
New faces in F1
Haas F1 will be making its F1 debut in Barcelona this week, while Renault returns to the sport after taking over financially strapped Lotus.
Hass, the first American-led team in F1 since 1986, launched its car on Sunday with the goal of "scoring points" and being "respected by fans and other teams."
Team principal Guenther Steiner said: "Everything is new for us. It's not only a new car, but a new team. In our first test of the season and first as a team, you try to make sure everything works as you designed it. You try to learn as much as possible about the car. You get the baseline on the car and you work off that baseline the rest of the year."
Renault had relinquished team ownership after the 2009 season and had become an engine supplier.
Renault racing director Frederic Vasseur said: "We know the road map and we have to deliver in the next three or so seasons."