Lewis Hamilton heads Nico Rosberg for a Mercedes one-two

Lewis Hamilton headed team-mate Nico Rosberg in first practice at the German GP as Mercedes' strong form continued.

Hamilton was 0.219 seconds quicker than Rosberg - and just over a second ahead of Red Bull's Mark Webber in third.

Force India's Adrian Sutil was fourth ahead of Lotus's Kimi Raikkonen.

Championship leader Sebastian Vettel was eighth fastest as title rival Fernando Alonso had a troubled session for Ferrari, failing to complete a flying lap.

The Spaniard's car ground to a halt out on the track on his very first lap and after the team repaired it and sent him out the same problem, believed to be with the electrics, recurred.

This time, Alonso managed to crawl back to the pits but he failed to get out for the rest of the session.

It was a bad start to the weekend for Alonso, who is 21 points behind Vettel in the championship and whose car was unexpectedly off the pace at the British Grand Prix last weekend.

Team-mate Felipe Massa was sixth fastest, 1.372secs slower than Hamilton. McLaren's Jenson Button was seventh and Force India's Paul Di Resta 10th.

The session was overshadowed by the ongoing controversy over tyres in F1.

The Grand Prix Drivers' Association (GPDA) - which represents all but three of the drivers - has said its members will "immediately withdraw" from the event if there is any repeat of the multiple tyre failures seen at the British Grand Prix.

In reality, if there is a failure, the drivers will seek to establish what has happened with tyre supplier Pirelli before making any decision about how to proceed.

GPDA chairman Pedro De La Rosa, the Ferrari development driver, said: "The point is if there is a blowout, we will think about it, look at the specific case and decide.

"You can have punctures, blowouts for many other reasons, like if there is a big piece of carbon fibre that goes through the tyre. That is part of racing. There is no problem with that.

"It is about being sensible and analysing what happens, but we really think what Pirelli has brought here is safe and they have done incredible work to bring it here in such a short time."

Pirelli has brought a new design of tyre to this race, with a Kevlar belt - the material that runs around the tyre and to which the tread is attached - replacing the previous steel one.

For the next race in Hungary in three weeks' time there will be a further change, with a reversion to the construction used last year.

First practice result.