Lewis Hamilton was feeling "positive" after his first full day of running at winter testing and said the W09 already felt like a "better car than last year".
The 2017 W08 clinched a fourth consecutive World Championship double for Mercedes but its 'diva' characteristics meant that it was far from the perfect car.
But Hamilton has said he felt an immediate improvement in its successor after setting the fastest time of testing so far, only on the medium tyres.
“I was really happy to get out, great to get on it,” Hamilton said.
“To get a feel for the new car and so far it is positive. I’ve only had 60 laps but it definitely feels like a better car than last year.
“Driving a new car is like trying to crack a new code: you have to figure out which different techniques you need to get the most out of it. But the W09 feels like last year's big sister.
“Of course we've got things to learn and things to improve on balance-wise but overall my feeling is positive.”
— Mercedes-AMG F1 (@MercedesAMGF1) March 1, 2018
Hamilton was also asked about his thoughts of winning a fifth World Championship but, at this stage, he is just focused on learning as much as he can about the new car.
He said: “My mindset is really about learning as much as we can and being as consistent and diligent as you can with the laps you do get, but of course this is all foundations, building blocks for the title fight."
Yet the Brit welcomed the challenges that would come with a closer title fight this year and would embrace being pushed to the limit.
"Definitely the closer it is the better it is when you win," he added.
"You want the guys you are racing with on a level playing field so you can really show the differences between us as drivers."
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