Fernando Alonso topped the times during a wet opening practice session for the Turkish Grand Prix in Istanbul, but it was a difficult start to the weekend for defending champion Sebastian Vettl - who ended the session with a trip into the barriers exiting turn eight.

The session gave teams the first chance to do meaningful running on the Pirelli wet tyres on a race weekend, and it took more than half an hour before anyone went out to set a representative lap time, with Alonso swapping positions at the tip of the time with the Mercedes of Nico Rosberg.

Alonso would ultimately end the session with the quickest time of 1min 38.670secs, which was well over a second quicker than Rosberg managed en-route to the second quickest time.

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Rosberg's team-mate Michael Schumacher was third quickest but it was an action-packed session for the seven-time champion who suffered a number of spins in the challenging conditions. Twice he managed to lose the car heading through turn eleven, the second time as conditions deteriorated towards the end of the session.

Renault pair Nick Heidfeld and Vitaly Petrov were fourth and fifth ahead of the Sauber of Kamui Kobayasi, with Felipe Massa, Daniel Ricciardo, Sebastien Buemi and Nico Hulkenberg rounding out the top ten.

For Vettel, the session would end in disappointing fashion when he lost control of the Red Bull coming out of turn eight after putting a wheel onto the kerb on the outside of the circuit. That tipped the car into a spin that saw him clout the barriers on the inside of the track, bringing out the red flags for some ten minutes while his car was recovered.

Having only completed four laps up to that point, Vettel ended the session well down the order in 17th place, six places behind team-mate Mark Webber who only completed five laps himself.

Pastor Maldonado also damaged his car at the end of the session with a spin at turn eleven - where a number of drivers aquaplaned wide - while Paul di Resta and Karun Chandhok where amongst those drivers to get away with spins without damaging their cars.

McLaren pair Jenson Button and Lewis Hamilton ended the session at the bottom of the times, with Button failing to set a representative lap and Hamilton the only man not to post a time at all. He was also prevented from going out to complete a practice start at the end of the session when a clutch issue meant he failed to exit the pitlane before it closed.