Right, that's us done for today. Keep an eye out for Andrew Benson's day one testing report going up on the website shortly.

Thanks for your company today and let's do it all again tomorrow.

1. Felipe Massa (Williams) 1:23.500 (103 laps)

2. Marcus Ericsson (Sauber) 1:24.276 (122)

3. Lewis Hamilton (Mercedes) 1:24.881 (48)

4. Daniil Kvyat (Red Bull) 1:25.947 (75)

5. Romain Grosjean (Lotus) 1:26.177 (75)

6. Kimi Raikkonen (Ferrari) 1:26.327 (80)

7. Carlos Sainz (Toro Rosso) 1:26.962 (86)

8. Jenson Button (McLaren) 1:31.479 (7)

And that's us done for today.

The first day of the final pre-season test ends with Felipe Massa quickest for Williams, ahead of Sauber's Marcus Ericsson and the Mercedes of Lewis Hamilton.

We'll wrap up all the times for you in a moment.

We're winding down in Barcelona. Ericsson, Kvyat, Grosjean, Raikkonen and Sainz are out on track.

Any late drama to come?

We sang Sauber's praises a short time ago and things are looking even better for the Swiss outfit now. Now shod with super-soft tyres, Marcus Ericsson's car has slotted into second after a 1:24.610.

Ok, it's probably on light fuel and is the equivalent of a goalkeeper making a save 'for the cameras', but what the hell.

Don't forget, this is a team that failed to score a point last season for the first time in their history. On this evidence a repeat of that misery in 2015 is unlikely.

Toro Rosso on Twitter: , external"We lost a bit of track time this afternoon, but team has done a great job working against the clock & we're out again."

Carlos Sainz's Toro Rosso ground to a halt a couple of hours ago with an unspecified problem.

Andrew Benson
Chief F1 writer

"Felipe Massa has set a new fastest time of the winter on Thursday with his pace-setting lap of one minute 23.500 seconds. But that is still not necessarily as impressive as the 1:24.881 Lewis Hamilton set in the morning.

"Let me explain. For one thing, Massa was on soft tyres and Hamilton on mediums. And the softs are reckoned to be 1.2secs a lap faster. Then there is the fact that Massa did only one flying lap, and Hamilton did his time on a 12-lap run.

"Assuming that each had only enough fuel for the run they did - a big assumption, admittedly - that means Hamilton could have gone about another 0.5secs quicker. Which would put him on a 1:23.0. And then bear in mind that Hamilton's team-mate Nico Rosberg did a 1:24.321 on mediums on Sunday."

It's been a good innings from Sauber today.

They started off the day with a lovely pic of the new C34 idling in a chilly pit lane (see 09:07), and have ended it with more than 100 laps in the bank and some strong times on the board to boot.

Marcus Ericsson has not put a wheel wrong today, ticking off 110 laps and counting. With a best time of 1:25.330, the Swede is currently third fastest, ahead of Red Bull, Lotus and Ferrari.

Image source, Getty Images

While nothing drastic, today's gremlins are a significant setback for the Mercedes team.

After minimal disruption to their pre-season running so far, to lose an entire afternoon's running at this stage is far from ideal, particularly with Williams hitting their straps.

It's is particularly tough on Lewis Hamilton, who has lost crucial running time and will now just have one more day in the car before the action proper kicks off in Australia in a few weeks.

Image source, Getty Images

Mercedes on Twitter:, external "That's our day done. Problem diagnosed as MGUK failure. Crew working hard to make sure we're back in business tomorrow. #BackToWork #F1"

'MGUK' is the Motor Generator Unit - Kinetic. This is one of two energy recovery systems that comprise the ERS power-boost system.

The thought belatedly occurs that Ron Dennis's "devoid of all injuries" comment earlier was a classic of the Ron-speak genre.

That is all.

Image source, Getty Images

Image source, Getty Images

Kimi Raikkonen has quietly got on with the task for Ferrari today. The 2007 world champion has churned out 54 laps for the Scuderia without ever really catching the eye. His best time is nearly three seconds off pacesetter Felipe Massa, albeit on slower tyres.

Image source, Toro Rosso

Toro Rosso's Carlos Sainz blatted along happily for 59 laps today before a problem forced the STR10 to grind to a halt. He's yet to get back out with a little over an hour remaining.

Alec Byhurst: "The Williams pace is genuine, can't quite match Mercedes but they're a solid second. Australia always throws a strange result though."

carmic: "McLaren are still waiting to announce a title sponsor. Do you think we will see them in Australia in the yellow & black of the AA?"

Dan White: "I'm a little confused by McLaren Honda, haven't they had a whole year to get ready?"

James Docherty: "Continual coverage of McLaren 'testing woes' but Force India have 0 miles on their 2015 car. BBC F1 coverage currently a 1-note tune."

Michael Boxwell: "Great news that Manor Racing appear to be back. Can't wait to see their new livery, even if 2015 is a rebuilding year."

Here's where we stand with around 90 minutes left to run today.

Felipe Massa sits atop the pile with his soft-tyre-aided 1:23.500, having earlier set a time on the mediums that compared favourably to the hitherto dominant-looking Mercedes.

Special mention to Marcus Ericsson too. The Swede gets a biscuit and a pat on the head as today's endurance star, cracking a century of laps for Sauber.

1. Felipe Massa (Williams) 1:23.500 (77 laps)

2. Lewis Hamilton (Mercedes) 1:24.881 (48)

3. Daniil Kvyat (Red Bull) 1:25.947 (58)

4. Romain Grosjean (Lotus) 1:26.177 (46)

5. Kimi Raikkonen (Ferrari) 1:26.327 (45)

6. Carlos Sainz (Toro Rosso) 1:27.261 (59)

7. Marcus Ericsson (Sauber) 1:27.802 (100)

8. Jenson Button (McLaren) 1:31.479 (7)

Lewis Hamilton's absence from the track has been explained anyway.

Mercedes have tweeted:, external "Failure in the Hybrid system keeping us in the garage currently. Crew working hard to fix it as we speak."

Image source, Getty Images

We'd like your views on what we've seen so far today.

What do you make of Williams' pace - are they a genuine challenger to Mercedes? What now for McLaren after another difficult day? And what of Ferrari and Red Bull?

Any and all comments welcome. Send them in using #bbcf1

"A different type of F1 refuelling stop. The transporters have been here in Barcelona for days, so their diesel fuel tanks need to be refilled in order to run the generators that power the lights and auxiliary equipment."

Image source, Knutson

After coming close to the best medium-tyre lap time of the two Barcelona tests thus far, it's no surprise to see Felipe Massa strap on a pair of soft boots and go for the glory of the fastest time of the test overall.

The Brazilian's time of 1:23.500 is half a second faster than Lotus' Roman Grosjean managed in the first Barcelona test, which had stood as the fastest overall time, although set on the super-soft tyre.

Image source, Getty Images

Andrew Benson
Chief F1 writer

"Well, here's a surprise. Unexpectedly, Lotus have announced that Spanish female GP3 driver Carmen Jorda has joined the team as development driver.

"It is an echo of the situation at Williams with Susie Wolff, who had that title last year before being given an upgrade to test driver this season.

"Jorda will work in the simulator this year with the aim of readying herself for a test in the F1 car later this year."

Image source, Lotus