Belgian Grand Prix: Sebastian Vettel fastest in third practice

Sebastian Vettel posted the fastest time in the third practice session at the Belgian Grand Prix ahead of qualifying later Saturday. (Racecentre)

The three-time defending Formula One champion beat Fernando Alonso's time late in the session to clock a best of 1 minute, 48.327 seconds, putting him .105 clear of the Spaniard and .206 ahead of Red Bull teammate Mark Webber.

Lewis Hamilton again struggled in his Mercedes and finished 12th. In the afternoon, the British driver bids for his fourth consecutive pole position. Vettel's last pole was four races ago, at the Canadian GP.

Toro Rosso driver Jean-Eric Vergne showed good form to finish fourth in mostly dry and overcast conditions.

In Friday's second practice, both Vettel and Alonso sustained punctures to their rear-right tires, putting the spotlight back onto manufacturer Pirelli. In cooler conditions than Friday afternoon's sunny second session, there was less pressure on the tires and the third practice was incident free.

The punctures to the cars of Vettel and Alonso prompted fears that Pirelli's tires could once again be vulnerable following a series of high-profile failures this season - notably at the Spanish GP and the British GP - that led to a boycott threat from drivers prior to the German GP.

That forced under-pressure Pirelli to revert back to the 2012 construction twinned with their 2013 compounds at last month's Hungarian GP. The modified tires held firm in Budapest.

But Spa is a tougher track because its long, steep corners create more pressure and Webber was one of several drivers who expressed concern over Friday's two punctures and demanded an explanation.

Following a detailed examination of the circuit between turns 13 (Fagnes) and 15 (Stavelot) where the incidents occurred, Pirelli discovered that a metal skidplate was the cause of the punctures and that it was not a tire-related issue.

"No issues from our perspective," Pirelli motorsport director Paul Hembery said. "We've seen no signs of fatigue. From our point of view, there is no reason to be concerned."

Drivers started out on the hard compound and, after switching to the quicker mediums, Adrian Sutil posted the fastest time of 1:49.122. But others then made the same switch and Alonso easily surpassed that.

All three of Hamilton's practice runs have proved difficult, and this time he span his Mercedes at the Bus Stop chicane.

Ferrari driver Felipe Massa had trouble with his understeer on turn nine and almost hit the barriers, and Kimi Raikkonen almost bumped into Alonso at the Bus Stop, when Alonso was coming into the pits and Raikkonen was on his flying lap.