Massa hit the front early on Thursday afternoon, bolting on Pirelli's soft-compound tyre and scything down to a 1m 23.500s - the fastest time seen so far this year in Barcelona, and enough to leave him almost a second clear of the field at the chequered flag.
“It was a good day for us and the car behaved well,” said the Brazilian. “The FW37 never showed a sign of an issue and this has been the case since we arrived at the first test in Jerez. We are doing a lot of work and seem to be moving forward.”
Sauber's Marcus Ericsson, the day's busiest man with 122 laps, moved into second following late runs on the supersoft tyre, but his best was still 0.776s adrift of Massa.
“It was a productive day, and for me the best one of the pre-season so far,” reported Ericsson. “The morning went well until the rain came, which forced us to slightly change our plans. In the afternoon we planned a full race simulation, which was a very good exercise for the team to get valuable data. Also for me it was important to get a feeling for the 2015 tyres over a race distance with different fuel loads.”
Ericsson's surge bumped the Mercedes of reigning champion Lewis Hamilton to third. The Briton had set the morning pace courtesy of an early run on medium tyres, but was then restricted by a failure of the MGU-K which curtailed his afternoon running.
McLaren fared even worse, as their troubled pre-season continued with the discovery of a hydraulic leak Jenson Button's lap count failed to reach double figures. The Briton languished in eighth in the timesheet, almost eight seconds off Massa's pace.
"After just seven laps, Jenson noted a vibration and brought the car back to the pits, where we discovered a hydraulic leak that required a full power-unit change," McLaren said. "The time required to complete the change unfortunately meant that our running for the day was over before lunchtime."
Red Bull's Daniil Kvyat was fourth fastest, narrowly ahead of Lotus's Romain Grosjean - outright pacesetter in the previous test at Barcelona - who was evaluating a new front suspension geometry, and Kimi Raikkonen, who had a tricky session in the Ferrari.
“This wasn’t an easy day for us as we spent time chasing our tails on an annoying reliability problem which by the end of the day we managed to put to bed, but that interrupted our programmes for the day quite considerably,” explained technical director James Allison. “Now we can put what we learn today in place for tomorrow so to have a day much more like we want to have”.
Carlos Sainz Junior was seventh for Toro Rosso, as the team introduced radical revisions, including a new front nose, to the STR10. The Spaniard caused the second of two red flags when he stopped at Turn 10 early in the afternoon session, although Toro Rosso were able to get him back out for the final half hour of the day. Kvyat had triggered the first red flags when he stopped in the pit lane just a few minutes after the session commenced.
“It was very nice to drive the car fitted with the update package,” commented Sainz. “That meant much of my time was spent collecting valuable aero data for the engineers to study. Then towards the end of the morning, I managed to do a real run. I could immediately feel an improvement in the car, in terms of its overall grip and balance. That was a very positive and motivating feeling for the rest of the day.”
Testing continues on Friday, when Force India are aiming to run their new VJM08 for the first time. Nico Hulkenberg is primed for driving duties, with the team hopeful of clocking up the first miles in their 2015 machine in the afternoon.
Unofficial Thursday testing times from Barcelona:
1. Felipe Massa, Williams, 1m 23.500s, 103 laps
2. Marcus Ericsson, Sauber, 1m 24.276s, 122 laps
3. Lewis Hamilton, Mercedes, 1m 24.881s, 48 laps
4. Daniil Kvyat, Red Bull, 1m 25.947s, 75 laps
5. Romain Grosjean, Lotus, 1m 26.177s, 75 laps
6. Kimi Raikkonen, Ferrari, 1m 26.327s, 80 laps
7. Carlos Sainz Jr, Toro Rosso, 1m 26.962s, 86 laps
8. Jenson Button, McLaren, 1m 31.479s, 7 laps