Sebastian Vettel showed the way in P2 at Spa-Francorchamps. (Photo: Getty Images)

showed the way in P2 at . (Photo: Getty Images)

Sebastian Vettel posted the fastest time before his rear tire punctured in Friday’s second practice at the .

The three-time defending champion had to stop with about 20 minutes of the session remaining when his right rear tire popped and the loose rubber flapped around as he made his way back to the pits.

The German had earlier clocked 1 minute, 49.331 seconds, which was .059 quicker than Red Bull teammate Mark Webber and .818 faster than Lotus driver Romain Grosjean. While it was not immediately clear what caused the puncture, it could be unwelcome news for Pirelli given the furor surrounding its fast-shredding tires throughout the season. With his session over, Vettel stood alongside officials in the garage as they studied the car’s data.

Tires ripped apart easily at the Spanish GP, and then blew up at Silverstone, leading to the threat of a boycott from drivers before the German GP. This prompted Pirelli to revert back to the 2012 construction twinned with their 2013 compounds in Budapest.

The modified tires held firm at the Hungarian GP last month, but Spa is a tougher track because its long, steep corners create more pressure. was fastest in the morning’s rainy first practice and seventh in the afternoon.

As the sun came out and dried the track, Vettel set the early pace, while Alonso was held up by traffic. Moments after Vettel’s puncture, Dutch driver Giedo van der Garde lost control coming out of a turn and slid across the track into the crash barriers. He climbed out unharmed but there was damage to the right of his Caterham.

Hamilton also had a difficult afternoon, finishing 12th. He asked mechanics over race radio “is my dashboard telling me the right time? Because the car’s all over the place.” Earlier on, Caterham drivers Charles Pic and Van der Garde got in each other’s way at the Bus Stop chicane and went off track. When Pic returned, he drove into the path of Vettel, who broke hard to avoid contact.

In the overcast morning, Alonso gave struggling Ferrari a boost. The two-time former champion is without a win in five races and has disappointed in qualifying all season, failing to even place on the front row. But the Spaniard clocked a best lap of 1:55.198. Vettel was sixth quickest.

They set their best times when the track was at its driest, and many drivers stayed in their garages as the rain started to fall again late in the session. Hamilton, who won the last race in Hungary, was 15th.

Conditions are often wet in Spa — the longest circuit on the calendar at 7.004 kilometers (4.352 miles). Because of its size, different microclimates mean it can be raining on one part of the track and sunny at another. Corners such as the steeply rising Eau Rouge and Blanchimont push drivers and tires to the absolute limit.

In the morning, it was dry at the bottom of the circuit but damp near the finish line. On Thursday, Alonso played down a rift within Ferrari by saying that a recent public rebuke from president Luca Di Montezemolo was a misunderstanding.

Alonso insists he is fully committed to Ferrari and hopes the varying demands of the next races offer chances to pressure Vettel. “We have Spa, medium downforce; Monza, light downforce; Singapore, maximum downforce,” Alonso said. “Who will catch him or put him in some difficulties, we will see in the next races.”

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