"This is testing, give us time - it is fine".
He was nearly 0.4 seconds ahead of Lewis Hamilton, who used ultrasofts on his quickest lap, and covered an impressive 165 laps.
"There is nothing fundamentally wrong on the auto and all the issues we had they are well under control", he added.
"There's clearly no doubt about that. there's still some blurriness, it's still not possible to say for sure whether we've got our nose in front of them or they've got their nose in front of us".
After snow and rain hit last week's first test, dry conditions and bright skies greeted teams with some gusts of wind later on.
Times in Barcelona have been far quicker this year than in the past, even eclipsing the efforts from last year's pre-season testing.
Vandoorne ended the day with 38 laps, the fewest among the 13 drivers who were on the track.
While the order may vary, Toto Wolff expects the three teams which raced clear of their rivals a year ago to fight for supremacy again in 2018. But that would be as good as it would get as an oil leak caused Alonso to come to a stop at the side of the track just over two hours into the day. I think it will be all three of us against each other.
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Three men reportedly ambushed Mhlengi Gwala and attempted to hack off both of his legs, TMZ reports. They had a handsaw and went for one of his legs‚ which is the concerning part.
Carlos Sainz was sixth fastest in the factory Renault auto - less than 0.2secs behind Alonso but using the medium tyre, which is well over a second slower than the hyper-soft.
"But if Australia was tomorrow, it's OK".
The midfield hodgepodge looks set to sustain with the likes of Williams, Force India and Haas all scrapping together with McLaren also in the fray.
"I think Mercedes do have a little bit on us, but I don't think it's too much".
"I think we made a good step with the vehicle today, again we made a few changes this morning and each change seemed more or less positive, or more positive than negative, so I think we got the auto running better today, got it faster, reliability was good, so it's getting there".
"As winter testing goes on you get a better and better idea of where things stack up".
"I haven't been through the data yet as I've just jumped out of the auto".
"I have a more positive than negative feeling for sure, it's on the right side of the spectrum where we need to be. It's the right thing to do; we have found some that are not fit for objective and that's great".