SPA, Belgium (AP) -- Sebastian Vettel had the fastest time before his rear tire punctured during Friday's second practice at the Belgian Grand Prix.
The three-time defending Formula One champion had to stop with about 20 minutes left. His right rear tire popped and the loose rubber flapped around as he made his way back to the pits.
The German was earlier timed in 1 minute, 49.331 seconds. That was 0.059 seconds quicker than the time of Red Bull teammate Mark Webber and 0.818 faster than Lotus driver Romain Grosjean.
It was not immediately clear what caused Vettel's puncture. But it could be unwelcome news for Pirelli given the commotion surrounding its fast-shredding tires this season.
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