Lewis Hamilton says Mercedes' W10 is "coming together", but the Brit has urged his team to keep on pushing in light of the opposition's performance.
Hamilton concluded his first week of pre-season testing with 58 laps in the morning session, but the cold and crisp conditions made it a tricky start of the day for the reigning world champion.
As the track warmed up, Mercedes undertook a series of runs on Pirelli's softer tyres to gauge the W10's grip and balance on the specific compounds.
"It was quite cold in the morning which meant that it was a little bit difficult with the tyres for everyone," said Hamilton.
"In general, the car was feeling better today and is coming together. However, we still have to keep on digging deep, analysing the data and the changes that we are making.
"The reliability has been really solid which is a great showing of all the hard work that everyone in the factory has done.
"But we have to keep on pushing, the competition looks very strong."
©Mercedes
Valtteri Bottas who ran in the afternoon, and also trialed the softer rubber, brought the Silver Arrows outfit its fastest lap of the week, with more performance to be extracted from next week's programme.
"We've been making progress throughout the entire week, the car felt better every single day," said Bottas.
"We're now looking forward to next week when we will hopefully make even more progress.
"Many thanks to the entire team, it was a good first test. Now we're all hungry to improve even more in the second test."
Gallery: The beautiful wives and girlfriends of F1 drivers
Keep up to date with all the F1 news via Facebook and Twitter
