Ahead of today's opening session, the air temperature is 17 degrees C, whilst the track temperature is 20 degrees. It is bright and sunny.

Tyre choices are medium (prime) and soft (option), as in Hungary , though the two tracks have little in common.

As ever, eyes will be on the skies and weather screens this weekend, this region notorious for its sudden weather changes which might just one part of the circuit.

There are no major changes to the track this year other than additional kerbing aimed at drivers taking advantage.

There are two DRS zones, one on the run up the hill to Les Combes (T5), the other on the short pit straight heading into La Source (T1).

After the three week break the teams and drivers are raring to go, and where better than this superb track. The updates are in abundance, with Honda expecting much from its updated power unit.

There is only one reserve driver on duty this morning and that's at Lotus where Jolyon palmer is in Romain Grosjean's car. The Enstone outfit broke the curfew last night for the second time this year. However, as teams are allowed two exceptions there will be no further action taken.

The lights go green and Rosberg heads the queue as the second half of the season gets underway. His Mercedes is fitted with a new front wing whilst Hamilton has a quite radical rear wing.

After four minutes all bar Ricciardo and Hamilton have made an appearance. The rest are safely back in their garages, their installation laps completed.

The Mercedes crew does its best to hide Hamilton's rear wing, whilst at Williams similar efforts are underway to shield the new front wing on Massa's car.

The lack of activity suggests that some of the teams and drivers think they are still on holiday, there are only five or six drivers seemingly doing any work. Of those, Sainz is the first driver to post a time, the Spaniard crossing the line at 1:54.645. moments later teammate Verstappen posts 54.836.

As Vettel begins his first flying lap, Stevens goes third with a 2:02.296.

Verstappen raises the benchmark with a 53.399 but this is soon eclipsed by Hamilton who stops the clock at 52.192. Finally we have some action. Raikkonen goes fourth and Vettel seventh, the German clouting the new kerbing at Raidillon.

Massa complains of a lack of power, whilst Rosberg reports a "loss of power". That's 'lack of' and 'loss of', two entirely different things. Subsequent TV footage shows Rosberg visibly slowing in Blanchimont. The German is told to head back to the pits but not to change gears.

Hamilton improves to 51.993 whilst Vettel goes fourth with a 53.100.

Button complains of a lot of understeer whilst mechanics swarm over Rosberg's car in the Mercedes garage.

A nasty little wobble for Alonso as the Spaniard doesn't appear to be suffering the same understeer issues as his teammate.

Another improvement for Hamilton (51.340), as Button complains that it is "getting very hot" on the left-hand side of his car.

As the opening half-hour comes to an end, there is a crowd of mechanics and boffins around the rear of Rosberg's car. Whatever the issue, it looks serious.

As the free tyres are handed back, Hamilton is quickest, ahead of Ricciardo, Kvyat, Nasr, Raikkonen, Hulkenberg, Vettel, Verstappen, Perez and Massa. Rosberg is the only driver yet to post a time.

A rare sight in the Red Bull garage... Helmut Marko is laughing.

After more than ten minutes of total inactivity, Maldonado and Raikkonen head down the pitlane.

The Finn soon improves, posting 51.478 to go second, just 0.138s off Hamilton's best. Maldonado also improves, the Lotus driver now up to fifth (52.539).

Seemingly, Rosberg's issue wasn't as bad as it looked for the German is now on track, albeit driving very, very slowly. "Fingers crossed," tweets Mercedes. Elsewhere, Hamilton goes quickest in S1.

Maldonado runs very wide in T9 and Rosberg at the same corner moments later.