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Valtteri Bottas is the first driver to dip below 1:22 as he does a 1:21.843.

Meanwhile, Force India's Nikita Mazepin has done his first time in testing - a 1:39.186 - before swiftly knocking six seconds off that with 1:32.967 to go eighth.

Andrew Benson
BBC Sport’s chief F1 writer

Spain’s Circuit de Barcelona-Catalunya is not picked for winter testing solely because it is the closest F1-spec track with the most reliably predictable weather at this time of year. It is also because its combination of long-duration corners tests every facet of a car’s ability, so gives the teams an unusually good insight into the performance of their machinery.

This year, though, there is an additional challenge, in that the track has been recently resurfaced - meaning the teams will have to work extra-hard to extrapolate what they see over the next few days. Mercedes technical director James Allison says: “Barcelona is quite an aggressive track on the tyres. It tears them up in the winter. You get a lot of graining. This year is going to be particularly interesting because they have just resurfaced Barcelona and it is lot smoother, and the sort of rubber that would be torn up last year in winter testing could probably survive this year. But that doesn’t necessarily mean it is going to be super-representative of the remainder of the year.”

Image source, Getty Images

Almost two hours in and we've got two more drivers on the timesheet as Romain Grosjean does a 1:28.415 to go sixth quickest before a 1:37.351 for Lance Stroll puts him eighth.

I'll do a full run down of latest lap times, tyres etc on the hour every hour.

And Valtteri Bottas lowers the benchmark slightly with 1:22.435. He's also on mediums.

Nico Hulkenberg, on medium tyres, is up to third with 1:23.854s. That puts him ahead of Fernando Alonso, who is currently back in the McLaren garage.

Fantasy F1 League: That's no wheel bearing - it's a flux capacitor.

Image source, Getty Images

Andrew Benson
BBC Sport’s chief F1 writer

McLaren say it will be a couple of hours before Alonso is back on track while they repair some of the damage his off caused to the car. They're investigating the reasons for it.

Image source, Getty Images

Fernando Alonso certainly appears to be holding something in the above picture, which would marry with the below tweet.

There are some real Columbos out there.

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CJC: Slightly to my surprise I'm kinda liking the halo - has a brutal, madmax kinda vibe to it - from certain angles at least.

Lord Roger: Having seen all the cars with the Halo on, I don’t actually think it’s that bad.

Image source, Getty Images

Here's a close up of the Red Bull with its official 2018 livery and the halo in all its glory.

My 30 second scan of social media suggests opinion on it has swung from "awful" to "not bad".

David Barkley: Can you add tyre choices beside the lap times for testing?

Ask and you shall receive, David. The tyres should be added next to the times below now.

Andrew Benson
BBC Sport’s chief F1 writer

Image source, Getty Images

Force India's new reserve driver, the well-heeled Canadian Nicholas Latifi, has tweeted this morning that he will not be taking part in pre-season testing this year.

He said: "A serious infection put me in hospital last week and I'm now recovering at home."

He says he is "aiming to be back on track in the next four weeks", which sounds like he will not be able to run at all in pre-season testing.

He was due to drive on Thursday this week. Latifi says he is "disappointed".

Actually, scrap that. Nicholas Latifi won't be in the car on Thursday...

Nikita Mazepin, by the way, is Force India's Russian development driver. He's got the whole day in the car today with Esteban Ocon and Sergio Perez taking over for the next two days before Canadian reserve Nicholas Latifi climbs in on Thursday.

Just one car on the track at the moment with Brendon Hartley steadily improving in the Toro Rosso. He's just done a 1:29.810.

Marcus Ericsson, Romain Grosjean, Lance Stroll and Nikita Mazepin are the only drivers yet to set a time.

Fahad Khan: Should we restart that "McLaren has broken down" page again or is it too early for that

Samu: Alonso's wheel came off? How can a team with the experience of @McLarenF1 make such a mistake on the 1st practice day?

Alex Withington: Looks like something mechanical broke on the @McLarenF1, external - wheel bearing or similar?

Valtteri Bottas, Nico Hulkenberg and Brendon Hartley jump straight back into their cars to embark on a bit more lappage.

Here's the times so far with just over one hour gone in testing:

1) Bottas (Mercedes) - 1:22.952 - Medium

2) Alonso (McLaren) - 1:24.202 - Super-soft

3) Ricciardo (Red Bull) - 1:27.214 - Medium

4) Hulkenberg (Renault) - 1:28.759 - Soft

5) Hartley (Toro Rosso) - 1:33.838 - Medium

6) Raikkonen (Ferrari) - 1:58.273 - Medium

Andrew Benson
BBC Sport’s chief F1 writer

And that is not a good start to testing for McLaren. Rid of Honda, they will have been expecting to log some serious miles and then, less than an hour into testing, Fernando Alonso ends up in the gravel at the last corner with the right rear wheel off his car.

We don’t know what happened yet, but it looks very much like a mechanical failure of some kind. Alonso crouched down beside the wheel after getting out of the car and then walked back to the pits carrying what looked like a wheel bearing. McLaren will have to get to the bottom of that before they can send him out again.

His progress? He was about to set the fastest time of the day so far - on super-soft tyres - on that lap.