Pirelli's Formula One tyres came under renewed scrutiny on Friday after triple world champion Sebastian Vettel suffered a blowout in second practice at the Belgian Grand Prix.
The Red Bull driver was fastest in the session but the sight of him nursing his stricken car back to the pits at slow speed with strips of right rear tyre buckling and flailing was more of a talking point.
Pirelli, who had to strengthen their tyres after a spate of blowouts at the British Grand Prix in June threw the sport into crisis, said Vettel had suffered a puncture and they were investigating what caused it.
Last month's Hungarian Grand Prix, the last race before the summer break, was free of tyre-related incidents but the Hungaroring is one of the slowest circuits while Spa is the second fastest track with heavy loads on the tyres.
Vettel had earlier lapped with a best time of one minute 49.331 seconds with the sun coming out after a damp start to the day in the Ardennes forests where the weather is notoriously capricious.
The championship leader's time looked ominous, 0.059 quicker than second placed team mate Mark Webber and comfortably clear of Frenchman Romain Grosjean in the Lotus with a time of 1:50.149.
Ferrari's Fernando Alonso set the largely unrepresentative pace in the opening session with a lap of 1:55.198 as Formula One drivers shrugged off the long August break and got back up to speed.
The Spaniard, third in the championship after 10 of 19 races, was ahead of the two Force Indias of Paul Di Resta and Adrian Sutil with the top three setting their best times in the final half hour of an incident-free session.
The afternoon was more lively, with Dutchman Giedo van der Garde losing control and crashing his Caterham into the barriers at Stavelot.
The McLarens of Briton Jenson Button and Mexican Sergio Perez showed signs of progress, despite the team backing away from earlier assertions that they could challenge for a victory this weekend.
Perez was fourth fastest in the morning and eighth in the afternoon, when the team tried out some developments for 2014. Button, last year's winner in Spa, was out of the top 10 in both.
Vettel, who has a 38 point lead over Lotus's Kimi Raikkonen, had been sixth fastest behind fellow-German Nico Rosberg in the Mercedes in the morning.
Raikkonen, who cried off sick on from media engagements on Thursday after weeks of speculation about his future and talk of moves to Red Bull or Ferrari, was sixth in the afternoon.
Mercedes' Lewis Hamilton, winner in Hungary and increasingly looking like Vettel's biggest rival after three poles in succession, had a quiet day and was 15th and 12th respectively for Mercedes.
Finland's Heikki Kovalainen, dropped by Caterham at the end of last year, made an appearance for the team in place of regular race driver Charles Pic in the opening session and was 16th quickest.