Lewis was first to climb aboard the W09 and take her for a spin bright and early on Wednesday, with the four-time World Champion taking on the morning shift in Spain.
He quickly got to work and spent a large chunk of the session at the top of the timesheets, before slipping to second place just before the lunch break. He remained there for the duration of the day.
The Brit put in 90 laps of the Circuit de Barcelona-Catalunya. His best lap of 1:18.400 on the Ultrasoft tyre was 0.749s quicker than his 2017 Spanish Grand Prix pole lap.
"Today has been positive," he explained. "The car is feeling good and the tyres are working. It's nice to have some good weather so we can really try and get in the mileage.
"The reliability has been good so far, which is a sign of all the great, hard work that the team has put in over the winter.
"It's still a work in progress, but it's definitely going in the right direction. The team's working hard - both here and back at the factory - to continue to climb the upwards gradient which we're currently on.
"I'm just so proud that I'm driving a creation that these guys have all worked to the bone to create. I'm proud to be the one who gets to go out there and try to exploit this car and bring them the results they deserve."