Ahead of today's opening practice session the air temperature is 15 degrees C, whilst the track temperature is 24 degrees.

As the Hamilton/Rosberg battle for the title continues the big news is that two teams are further succumbing to the ever rising cost of F1. Kamui Kobayashi is dropped in favour of 32-year-old Sports Car veteran Andre Lotterer and Max Chilton 'stands down' in favour of American Alexander Rossi.

Whilst both teams are playing down the financial significance of all this it will be interesting to see who might drive in some of the subsequent races particularly as Caterham appears to be favouring 'locals'.

The only other driver replacement is at Sauber where Giedo Van der Garde is in for Esteban Gutierrez.

There are two DRS zones, the detection point for the first is 240m before Turn 2, with the activation point 310m after Turn 4. The second detection point is 160m before Turn 18, with the activation point 30m after Turn 19.

In terms of tyres, Pirelli is providing the medium and soft compounds though we've a sneaky feeling the full wets and Inters will make an appearance at some stage over the next 72 hours.

The lights go green and, unsurprisingly, Lotterer is one of the first out, along with Ericsson, Bottas, Bianchi, Vergne and Magnussen.

Other than the works outfit, the Mercedes customer teams are going to be strong here, with many tipping Williams to be the Brackley outfit's strongest challengers.

One by one the drivers emerge, watched by Chilton on the Marussia prat-perch, subsequently heading straight back to the pits, despite the extra set of tyres which is meant to guarantee some extra running. Last out are Hamilton and Hulkenberg.

Around fifteen minutes into the session the drivers begin returning to the track, Button and Magnussen among the first.

Button posts 1:54.221 but this is quickly beaten by teammate Magnussen who stops the clock at 53.405. Moments later Bianchi posts 57.599 to go third.

Having posted 55.878 to go fifth, Rosberg goes quickest in S1. The German crosses the line at 52.143 while Magnussen, who looked set to improve, has a poor final sector.

Hamilton goes third with a 53.55, ahead of Bottas, Button, Perez and Kvyat.

Meanwhile, it is claimed that following a late application to the stewards, Chilton will race this weekend.

As Alonso goes second with a 51.397, Rosberg ups the ante with a 51.724. Only Grosjean and Maldonado have yet to post a time.

Replay shows Alonso slowing on the run down to Eau Rouge and using the escape road where he stops and then gets going again.

As Grosjean posts 56.161 to go 16th, Vettel goes fifth with a 53.369. Hamilton currently eighth and Ricciardo tenth.

A big lock-up in the final chicane for Hamilton who heads straight into the pits. Moments later Rosberg makes the same mistake but opts to stay out.

The first half-hour comes to a close, at which points the 'free' set of tyres are handed back.

Rosberg is quickest, ahead of Alonso, Bottas, Magnussen, Vettel, Perez, Hulkenberg, Hamilton, Button and Ricciardo.

As Maldonado heads out, having missed out on the 'free' tyres, mechanics are swarming over Button's car.

Maldonado, the only man on track, posts 56.359 to go seventeenth.

Despite not having driven an F1 car for some years, Lotterer (59.640) is ahead of Rossi (2:00.383), though the American is probably feeling a little demotivated after his team's U-Turn.

Maldonado, still enjoying an empty track, improves to fifteenth with a 55.346.

As the Venezuelan heads back to the pits his place on track is taken by teammate Grosjean who is currently sixteenth (55.672). The Frenchman subsequently improves to ninth with a 53.597.

Finally, there is movement in the pitlane as Rosberg, Vergne, Ericsson and Lotterer head out to join Grosjean.