Ferrari resurgence appears to be no flash in the pan as Kimi Raikkonen and Sauber's Marcus Ericsson top the timesheets at Jerez.

While the day began on a damp track following overnight rain, it soon dried out and by the afternoon conditions, though still a little cool, were almost perfect. Consequently, the eight drivers on duty were able to complete 655 laps between them.

Once again, the Ferrari SF15-T never missed a beat, and in the process of adding another 106 laps to the team's tally Raikkonen posted the fastest time of the week, while teammate Sebastian Vettel's 1:20.984 drops him to second.

To further compound the Maranello manufacturer's pace and reliability, Marcus Ericsson, in the Ferrari-powered Sauber ended the day second quickest, teammate Felipe Nasr having been quickest yesterday.

The frustration of Caterham must seem a lifetime ago for Ericsson, the Swede putting in a solid performance today. Bookending the session perfectly, he posted the first time of the day then brought the session to an early close when he ran out of petrol and brought out the red flags with less than ten minutes remaining.

Mercedes is hardly on its back foot however. On his way to posting the third best time of the day, Lewis Hamilton completed another 116 laps, bringing the Brackley outfit's total for the four days to over 500.

There was a minor scare when the Briton spun just over two hours into the session following a technical failure, causing the second red flag of the day. However, he was soon on his way again.

Whilst it was another quiet, steady day for Williams, which had Felipe Massa on duty, Max Verstappen and Toro Rosso continue to impress.

After a strong start to the day, Romain Grosjean, making his debut in the Lotus E23 was side-lined in the afternoon with a power unit problem. "An early bath for us after 53 laps today," said the Enstone outfit.

"The data suggested something was amiss with the power unit," it added. "A productive first test nevertheless."

The fact that said power unit is a Mercedes will have been noted elsewhere along the pitlane.

It was another difficult day for Daniil Kvyat and Red Bull. Stopping on track just a few laps into the session, thereby causing the first red flag of the day, the team subsequently traced the problem to an ERS issue. After three-and-a-half hours work by the Enstone crew the Russian was back on track, adding a further 55 laps to the 8 completed before the failure.

Once again, McLaren suffered a roller-coaster ride. After a couple of minor issues which saw the MP4-30 grind to a halt, once as it left its garage, another time at the end of the pitlane, Jenson Button finally began to string some laps together.

However, around ninety minutes before the end of the session the team reported a fuel pump problem. Although confident that it would get him out again, the Woking outfit eventually admitted defeat and revealed it would follow Lotus' example and take an early bath.

Of course, it's early days, but based on the last four days Mercedes could have a serious fight on its hands this season. While there remain the usual questions in terms of fuel loads and sandbagging, there is no escaping the pace and reliability of the Ferrari or indeed its similarly-powered rival from over the Alps.

Whilst Williams has yet to truly show its hand, the early signs are that Toro Rosso is much improved, even if its sister team is struggling.

With two weeks before the cars fire up in Barcelona, you can rest assured that the teams will be working flat-out and burning the midnight oil as they analyse the lessons learned this week.

"Over the winter, we have made a good step forward and this car is definitely an improvement over the 2014 one," said Raikkonen, who spent the day working on tyre evaluation and set-up work. "It's the whole package that has progressed, but there is still a lot of work to do. We are not comparing ourselves to the others, we just got on with our own job, without looking at what they were doing.

"Times are of relatively little importance," he continued, "what matters is that we have put together a good number of laps without having any real problems. Today's conditions were not that easy, with too strong a wind blowing on track, but then it's the same for everyone. As I said, we now have a good basis from which to start the development work."

"When you bring a new car to the track, you're always excited and worried and that is mixed with a profound hope that all the hard work of so many people will be rewarded with performance," admitted James Allison. "We leave this test without having all our expectations met, simply because we would have liked to have done even more mileage and because you always want to be faster. However, having said that, given how it went, overall we can be pleased, because we have had a solid start. Now, we are keen to push on with the development work in Barcelona, so as to be ready for the first race in Melbourne."

"If one makes a comparison to last year, these past few days of testing have produced encouraging signs," smiled team boss Maurizio Arrivabene. "The team has worked well, both at home and at the track and our drivers did a great job, providing the engineers with valuable feedback. But above all, I am pleased that the group has rediscovered its motivation and team spirit.

"In terms of performance, I don't think our competitors – one in particular – have shown their true potential over these past days," he added. "I think we will only discover the truth about them and about ourselves at the last Barcelona test session. Now, my attention turns to Paris, where tomorrow's meeting will decide the future of our sport."