Ahead of today's practice session, the air temperature is 26.1 degrees C, while the track temperature is 33.7 degrees. As was the case yesterday, it is bright and sunny.

Ferrari set the pace in both of yesterday's sessions, Vettel in the morning and Leclerc in the afternoon - but the longs runs suggest that Mercedes has the edge, and let's not forget the German team - like Honda and Renault - has an upgrade here this weekend.

Sergio Perez finished the day ahead of the Red Bulls, but a late engine failure will see the Mexican revert to a previous-spec engine.

The lights go green and Norris is first out, followed by Raikkonen and Magnussen.

As more drivers head out, there's a mixture of hards and mediums.

That said, Sainz heads out on softs.

Following a brief burst of activity in the form of install laps, the track falls silent and it is several minutes before the sound of a V6 fills the air, it is the Mercedes of Kubica.

As the pole begins a flying lap, he is joined by Leclerc and Albon.

At the line Kubica posts 1:50.877, watching in the pits, Frank Williams does not look particularly impressed.

Leclerc begins his first flyer, the youngster passing teammate Vettel on the Kemmel Straight.

Crossing the line at 44.789, Leclerc is 6.088s quicker than Kubica's Williams. Albon subsequently posts 50.864.

Quickest in the final two sectors, helped by a tow from his teammate through Blanchimont, Vettel raises the bar with a 44.657.

Grosjean goers third (47.065), ahead of Magnussen, Sainz and Kvyat.

Following the initial appearance of the mediums and hards, most drivers are now on the softs.

Magnussen told that he can re-set his "FUNC", indeed, he can "go back to FUNC 1". Shades of Sylvester.

Twenty-two minutes in, Bottas is the first of the Silver Arrows to appear.

A 45.630 sees Hulkenberg go third.

"I've lost my left mirror," reveals Gasly, "the glass is not on the car anymore."

As Bottas begins his first flying lap, Verstappen heads out, followed by Hamilton.

Bottas goes third (0.559s off Vettel's pace, while Hamilton goes fourth (45.566), almost a second down, and Verstappen eighth (46.441).

Despite reverting to the previous spec engine, Perez maintains yesterday's form, the Mexican posting 46.003 to go fifth. Raikkonen is sixth (46.301), ahead of Stroll and Verstappen.

Out comes the yellows as Hamilton is off and into the barriers in Turn 12 (Stevelot).

"Sorry guys, I hit the wall," says the Briton, as the session is red flagged.

The crowed cheers as Hamilton climbs from the Mercedes.

Replay shows the Briton losing the back end of the car as he enters the left-hander, and spearing into the tyre wall. Unfortunately, the crowd cheers.

And with 21 minutes remaining, as the tyre wall is rebuilt following the extrication of the Mercedes, the stoppage is compromising the drivers' qualifying sims.

The flatbed truck brings the badly damaged No 44 back to the paddock, and so begins the frantic bid to get the car repaired for qualifying. The big question being, will it require a chassis change?

The session will resume with 13 minutes remaining.

Thus far, everyone has had some soft running, though Norris has only used the mediums thus far this morning.

The lights go green and Leclerc leads teammate Vettel back on to the track to get proceedings underway. Bottas is also keen to get back to work.

Of course, with so little time and so many drivers, there is bound to be a traffic issue.

Vettel's lap is compromised from the outset, the German locking-up at La Source.

Leclerc, who has been told he has only one timed lap, raises the bar with a 44.206, Vettel having aborted his lap following that lock-up.

Bottas improves to 44.703, while Gasly goes sixth with a 45.752, only to be demoted when his former teammate, Verstappen, goes fourth with a 45.312.

Currently propping up the timesheets is Albon, whose best time is 6.475s off the pace. He is on track, and has done 14 laps, which is about average, so Red Bull clearly has a plan.

"I don't know if we are doing something wrong with the brake balance," reports Verstappen, "it is so easy to lock the inside of the wheel."

A 44.974 sees Ricciardo go fourth, just 0.768s off Leclerc's pace.

Told there is no time for refuelling, Vettel is asked if he wants to stay out. Indeed he does. However, once again he aborts the lap in the second sector.

Sainz, currently 14th, is told that there is a tenth to be found in Turn 9.

Raikkonen and Giovinazzi go eighth and ninth, the Alfa pair clearly benefitting from the upgrade the works Ferraris had a couple of races ago.

The session ends. Leclerc is quickest, ahead of Vettel, Bottas, Ricciardo, Verstappen, Perez, Hamilton, Raikkonen, Giovinazzi and Gasly.

Grosjean is eleventh, Hulkenberg, Magnussen, Sainz, Norris, Stroll, Kvyat, Russell, Kubica and Albon.

All eyes on Mercedes now as we wait to see if the world champion will play any further part in today's proceedings.

Check out our Saturday gallery from Spa, here.