Lewis Hamilton say Mercedes' new W10 feels "quite a bit different" compared to the team's 2018 Formula 1 car after completing his first extensive running with it at Barcelona.
The car, which was shaken down at a filming day at Silverstone last week, completed 69 laps on the first day of pre-season testing on Monday morning with Valtteri Bottas at the wheel before Hamilton took over and logged a further 81 in the afternoon.
The pair finished the day eighth and ninth respectively of the 11 drivers who ran, close to two seconds slower than Ferrari's Sebastian Vettel, although Mercedes said it had not targeted ultimate lap time.
"It's good to be back in the car and it's great to see the car is running as smoothly at it is," said Hamilton.
"It feels quite a bit different compared to last year and we're now getting to know it, trying to understand the balance and trying to understand the characteristics we're working with.
"It's been a positive first day, we got through our running plan and there's lots for us to learn about the new car which we will be working on."
Bottas added: "Since the car is brand new, we immediately found ways to improve the balance and the behaviour.
"We will continue to work on that in the next days."
Technical director James Allison said the team had taken a relaxed approach to lap times on its first full day with the new car.
"We managed to squeeze quite a lot in to this first day of running, finding out both how the 2019 tyres are working and also learning about the way the car responds to set-up changes," he said.
"Today was not about absolute pace and we're reasonably pleased with the handling of the car and the way it is behaving."
Team boss Toto Wolff insisted that Mercedes would not be distracted by pacesetter Vettel's fast times straight from the off.
"You need to be disciplined," he said.
"We are all keen in looking at lap times, and obviously you want to be quick and on top of the leaderboard in every single session, but it's not the purpose of the test, it's [to] go through all the parts that you want to look at and go through what all the data and sensors tell you.
"We don't know yet how the competitive order is going to be, we feel no sense of entitlement for being top of the pack, and this morning showed you why.
"The Ferrari looked ultra strong."