Germany's defending world champion Sebastian Vettel topped the times and then dismissed concerns about his tyre failure during Red Bull's domination of Friday's second practice session for the Belgian Grand Prix.
Red Bull Racing's German driver Sebastian Vettel drives during the second practice session at the Spa-Francorchamps circuit in Spa on August 23, 2013 ahead of the Belgium Formula One Grand Prix. Vettel topped the times and then dismissed concerns about his tyre failure during Red Bull's domination of Friday's second practice session for the Belgian Grand Prix.
The 26-year-old, sporting newly-bleached hair, clocked a best lap of one minute and 49.331 seconds to outpace Red Bull team-mate Mark Webber by 0.059 seconds.
Overall, it was another demonstration of the defending triple world champion's mastery of the track and the conditions, until Vettel suffered his right rear puncture close to the end of a long run on medium tyres.
The incident immediately revived memories of the dramatic blowouts at the British Grand Prix in July, when Italian suppliers Pirelli were at the centre of a controversy over safety that led to threats of a drivers' strike.
Pirelli have supplied their hardest compounds to cope with the demands of the majestic seven-kilometre circuit and have also prescribed precise limits to camber and pressure settings - to avoid any punctures.
Vettel said he had not done anything excessive in his driving that he felt could lead to a puncture.
"I wasn't doing any more or less than I did for the 10 laps before...We don't know what happened yet," he said.
"Basically I lost the rear right very suddenly, so we need to have a look, but other than that it was a pretty positive day."
Webber said: "I think until we find out why it leaked or failed there is no real concern... They would have called me in if there was an issue, but it looked like a pretty standard failure."
Romain Grosjean was third-quickest for Lotus ahead of Felipe Massa of Ferrari as two of the leading teams' number two drivers upstaged their top men in a dry and warm session.
Jean-Eric Vergne of Toro Rosso followed suit by also outpacing his team-mate Daniel Ricciardo of Australia, who finished down in 16th.
Kimi Raikkonen, still tight-lipped on his future amid rumours about his intentions for 2014, was sixth in the second Lotus ahead of two-time champion Fernando Alonso in his Ferrari. Alonso was fastest in a wet morning session earlier in the day.
Lewis Hamilton, who scored his maiden win for Mercedes at last month's Hungarian Grand Prix, the last before the mid-season European summer break, was 12th while his former team-mate and compatriot Jenson Button finished up 15th for McLaren.
"I'm struggling -- the car doesn't feel as good as it did in the last race -- but I'm still hopeful," said Hamilton.
"I don't think the car is bad, I just think we need to refine the set-up. The Red Bulls are obviously very quick, but I don't think they're unbeatable."
Vettel and Webber were always in control of the session, which was run in much improved conditions that saw one accident when Dutchman Giedo Van der Garde crashed into the barriers with 15 minutes remaining in his Caterham. He was not seriously injured.