Fernando Alonso topped the opening practice session for this weekend's Belgian Grand Prix as Spa's infamously unpredictable weather played a major role.

A misty, damp start and early rain had left half the track wet and the other mostly dry, and after all the drivers have completed their installation laps, the wait was on to see how conditions would play out.

Early on it was the McLaren's setting the pace on the intermediates as Jenson Button led from Sergio Perez, though lap-times failed to break the 2:00 barrier.

With the threat of more rain around and a track not ready for slicks but not wet enough for the inters, it wasn't until the halfway point when Mark Webber bolted a set of hard compound slicks to his Red Bull and started lapping quite quickly.

Although his overall lap-time was still slower than those earlier set on intermediates, his middle sector was easily faster than the others and that started a flurry of activity as the driver's headed out before more forecast rain arrived.

The McLaren's remained fastest, Sergio Perez leading the way before eventually Fernando Alonso came out on top after a 10 minute spell that had seen lap times drop 5 seconds.

The Spaniard's 1:55.198 is still quite some way of the pace we are expecting at Spa, however it was enough to edge out the two Force India's of Paul di Resta and Adrian Sutil. Sergio Perez maintained fourth for McLaren as Nico Rosberg moved the Mercedes up towards the top of the timings.

In a session where gaining any sense of the pecking order was impossible, Sebastian Vettel's Red Bull was down in 6th, the damp conditions has not always sorted the championship- leading team and with the threat of rain throughout the weekend, Vettel will be hoping they find a solution.

The two Sauber's of Esteban Gutierrez and Nico Hulkenberg were 7th and 8th, while Daniel Ricciardo put all the off-track diversions to one side and was 9th for Toro Rosso. Valtteri Bottas's Williams completed the top 10.

The session was dominated by driver's struggling to slow down into the Bus Stop chicane and at La Source with two drivers, notably Sebastian Vettel having spins at the first corner.

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