Sebastian Vettel posted the fastest time before his rear tire punctured in Friday's second practice at the Belgian Grand Prix.

The three-time defending Formula One 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.

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.

"We don't know what happened when I got the puncture. We lost the rear right, very suddenly, so we need to have a look," Vettel said. "Pirelli is working on finding out why it happened.

"Other than that, it was a positive day. This morning was tricky with a few rain drops and the track was slippery. It's a lot colder here than Hungary, but that's Spa and part of this circuit."

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. That 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.

Pirelli's motorsport director, Paul Hembery, thinks the tires were not to blame this time.

"Both Sebastian Vettel and Fernando Alonso had a puncture on their right-rear tire around turn 14," late in the afternoon session, Hembery said. "From initial analysis it appears that both were caused by an outside factor, probably debris on the track."

Alonso was fastest in the morning's rainy first practice and seventh in the afternoon.

As the sun came out and dried the track, the Red Bulls set the early pace, while Alonso was held up by traffic.

"We had a pretty good run today but there's still some performance to find and we need to get the car a bit better balanced," Webber said. "It was good to have stable conditions this afternoon."

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, Van der Garde and Caterham teammate Charles Pic 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.