Ahead of today's opening session, the air temperature is 21 degrees C, while the track temperature is 29 degrees. It is very overcast and there has been some rain earlier this morning. Humidity is 72, which is unusually high for the region.

Tyre options this weekend are medium (prime) and super-soft (option).

In terms of drivers, Sakon Yamamoto replaces Karun Chandok for the second successive weekend, while Paul di Resta is driving Tonio Liuzzi's car in this session.

When the lights go green, di Resta is first out of the box followed by Sutil, Yamamoto. Buemi and Glock.

Three minutes into the session, Felipe Massa heads down the pitlane, the Brazilian making an emotional return to the track where his career - indeed, his life - almost ended last season. Because the track is rarely used other for the Grand Prix, the tyre marks where he went off last year are still clearly visible.

As is becoming the norm, Schumacher stays out for a second lap, Rosberg, Webber and Barrichello also.

As Trulli is pushed back into his garage, clouds of smoke billow from the Lotus. Elsewhere, Rosberg comes out for a second run, his team doing some early aerodynamic testing. After a couple more laps he pits then goes out again.

Seventeen minutes into the session, Massa posts the first time of the day, the Brazilian crossing the line at 1:38.521. Moments later Sutil posts 26.143.

Schumacher's car makes a horrendous noise as the German returns to the pits however, a couple of minutes later the German is back on track, so it's clearly nothing serious.

As several drivers, including Hulkenberg, appear to be carrying out aero tests, Alguersuari goes second (28.802), ahead of Senna, Yamamoto and Massa.

On his first flying lap Webber raises the benchmark with a 24.856, next time around the Australian crosses the line at 23.396.Alguersuari consolidates second place with a 24.666.

As Webber raises the benchmark ever higher (22.990), Vettel, having gone quickest in S2 and S3, posts 23.317 to go second on his first flying lap.

Thirty-six minutes into the session, Alonso heads down the pitlane as Webber posts a 21.710. Elsewhere, de la Rosa runs wide at Turn 11 while Button heads down the pitlane.

A 25.654 sees Alonso go eighth, while de la Rosa (23.212) and Schumacher (23.617) go third and fourth. Moments later, Rosberg leapfrogs his Mercedes teammate with a 23.471.

At half-time, it's: Webber, Vettel, Rosberg, de la Rosa, Kubica, Schumacher, Petrov, Alguersuari, Alonso and Buemi.

As Vettel posts 21.275 to go quickest, there are still no times from di Resta, Hulkenberg, Button, Trulli, Kovalainen or Hamilton.

With 43:10 on the clock, Hamilton heads down the pitlane. On his first flying lap the 2008 champ posts 24.075 to go fourteenth.

Kubica goes quickest in S1 ultimately posting a 22.228 to consolidate third position. Barrichello goes fourth ahead of his former teammate Schumacher.

Di Resta and Button are the only drivers yet to post a time, while the McLaren crew is still working on the Englishman's car, the Scot is understood to be waiting on a set of tyres. The Silverstone-based team was reprimanded in Germany for mixing up the tyres on its drivers' tyres. Elsewhere, Alonso improves to sixth with a 22.863.

With 34:30 on the clock, Webber goes quickest in S1, finally crossing the line at 21.177.

On-board footage shows the front wing on the Red Bull clearly flexing on the straights and then again under braking. The team and the FIA insist that this is fully legal, but some rival teams and sections of the media disagree. Ferrari has a similar wing, which it used at Silverstone and Hockenheim, but is undecided as to whether it will use it here.

With half-an-hour remaining, Button heads down the pitlane and Webber consolidates his top spot with a 21.106.