Ahead of today's qualifying session the air temperature is 26 degrees C, while the track temperature is 44 degrees. It is bright and sunny but also very windy.

Having set the pace in both of yesterday's sessions, this morning Lewis Hamilton could only manage third, while Valtteri Bottas, following two engine changes overnight, was fourth.

Leading the way was Kimi Raikkonen, with teammate Sebastian Vettel second, Ferrari pulling one of the big surprises of the season.

Whether the Italian team was sandbagging yesterday, or Mercedes this morning, remains to be seen. Certainly, in the battle of the updates yesterday sent a shiver down quite a few spines in the paddock and in the media room.

With this being the scene of winter testing, where Ferrari's improvement first became apparent, it is fitting that once again Barcelona might prove the form indicator.

Red Bull appears to have taken a step forward, but the initial signs are that it still isn't enough and that the Austrian team has much work still to do.

While the titanic battle for the title holds our attention, so too an equally fierce battle is raging behind for supremacy in the midfield. Over the course of the sessions thus far we have seen Haas, Renault and Williams - well Felipe Massa - all looking strong, with Toro Rosso also up there. Surprisingly, Force India hasn't been that impressive thus far.

On the other hand, full marks for effort and commitment to Fernando Alonso, who, following yesterday's nightmare, posted the tenth best time in FP3, teammate Stoffel Vandoorne's 19th place surely the better indicator of where McLaren stands at present.

Whatever want before, it is the next hour, particularly the final stages of Q2 and Q3 when we shall see the true pecking order, and whose upgrades have worked whose have some way still to go.

Following an issue late in this morning's session, Ferrari elected to change the engine in Vettel's car after discovering a water leak, which, apart from anything else, takes him that much closer to the penalty zone, this being his third engine of the season.

As we await the green light, Vettel sits quietly at the back of his garage, watching the crew work on his car.

Green light... but it is almost a minute before an engine fires up, it is that of Vandoorne. Vettel follows the Belgian down the pitlane, the German eager to get an installation lap under his belt.

It's the mark of the man that Vettel thanks his crew - in Italian - as he laps the circuit.

Sadly moments later, the German is told to "stop the car, stop the car now". Are you sure?" he asks. "Try to reach the box," he is told.

Meanwhile, Vandoorne has posted 23.219 to get things underway.

As Grosjean spins in T13, not only has Vettel not "boxed" he has gone quickest in S1. He maintains the pace in S2, finally crossing the line at 20.939 to go quickest... and moments earlier it seemed terminal.

As more drivers head out, Hamilton goes quickest in the first two sectors, crossing the line at 20.511. Bottas goes third with a 20.991.

A 22.390 sees Massa go fourth but he is instantly demoted when Sainz posts 22.082.

Despite a scrappy final sector, Ricciardo goes fourth with a 21.704, leapfrogged moments later by his Red Bull teammate who posts 21.430.

Raikkonen casually posts a series of PBs, crossing the line at 21.120 to take fourth.

Perez goes seventh (21.998) as Vettel and Bottas head out again even though both seem secure. Then again, in the German's case he is probably acquiring some data for the team.

Alonso goes 12th (22.345), and with 4:3 remaining the drop zone comprises Grosjean, Stroll, Palmer, Magnussen and Kvyat.

Raikkonen also goes out again, this time posting 20.742, just 0.231s off Hamilton's pace.

Grosjean improves to 7th with a 21.822 while teammate Magnussen goes 8th.

As the clock ticks down the bottom half of the timing screen is green as drivers post a series of PBs.

Alonso goes tenth while Hulkenberg can only mange 12th. Stroll fails to improve as do Stroll and Palmer.

A late improvement for Massa proves costly for Ericsson who misses the cut, just...

Quickest is Hamilton, ahead of Raikkonen, Vettel, Bottas, Verstappen, Ricciardo, Grosjean, Ocon, Sainz and Magnussen.

We lose Ericsson, Palmer, Stroll, Vandoorne and Kvyat.

Making the cut by the skin of his teeth, Massa admits he was struggling to make his tyres work.

Check out our Saturday gallery from Barcelona, here.