Williams has leapt back into contention as a likely 2015 force.
After the first two tests at Jerez and Barcelona, it was suggested that in light of the Grove team's low-profile performance, rivals might be set to overtake Williams as 'best of the rest' behind Mercedes this year.
But as upgrades were added up and down pitlane on Thursday as the final test before Melbourne began, it was Felipe Massa who set the headline time.
His laptime, in fact, was easily the quickest of all the Barcelona running so far.
"I think we needed to prove that we are there in the fight," the Brazilian said afterwards.
"It's still difficult to be sure where we are, especially compared to Mercedes," he added.
"We will have to wait until all the drivers are on the track at the same time with the same amount of fuel and the same tyre," Massa is quoted by Globo.
"But as far as the other teams, we are there," he insisted. "Ferrari did not have a good day today, I do not know what they were doing, but they have improved, no doubt.
"And the race simulation by Red Bull was not very good," Massa added.
"I still believe Mercedes has the best car."
Like Ferrari and Red Bull, also struggling on Thursday was Mercedes, whose running with world champion Lewis Hamilton was limited due to a MGU-K issue.
Team boss Toto Wolff is expecting the German marque to have a tougher 2015.
"When you are in front," he is quoted by La Presse, "it is very difficult to keep this advantage.
"So far I see a more balanced spread in terms of performance. Ferrari has improved," said Wolff, "and we will see what McLaren and Honda can do.
"We are the team to beat," he acknowledged. "We have a target on our back and everyone is shooting at us."
That applies even to the midfield teams. Indeed, looking at the nine teams in action on Thursday, it is hard - McLaren aside - to identity who is lagging behind.
Toro Rosso turned up on Thursday with a substantially revised car compared to the first Barcelona test.
But "Everyone has become much better," rookie Carlos Sainz remarked. "Lotus, Sauber, us. So it is very difficult to say where everyone is."
The team that is obviously struggling is McLaren-Honda.
Mercedes, for instance, soared past the 1000 lap mark for the winter period on Thursday, while the newly Honda-powered MP4-30 has done just 210 overall.
"That's an average of only 23 laps per test day," said Oskari Saari, a commentator for the Finnish broadcaster MTV3.
"At the moment it is difficult to imagine that even the legendary names of McLaren and Honda can in any way be ready when the racing starts very soon in Australia," he added.
Team supremo Ron Dennis was trackside on Thursday, and with star driver Fernando Alonso resting after his bizarre crash and hospitalisation, McLaren's problems are piling up.
"We have an engine problem now," Dennis revealed to reporters on Thursday.
"I am not sure how it will unfold over the next few days," he admitted. "We are still at the foot of a steep mountain."