Nico Rosberg may have been fastest in both free practice sessions on day one of the Belgian Grand Prix at Spa-Francorchamps, but he admitted that he was lucky to have walked away from a scary high-speed accident in the afternoon.

The German was already fastest in FP2, having improved on his morning time after fitting the softer Pirelli tyre, but, having completed just eleven laps on the option rubber, suffered a blow-out on the run to Blanchimont that sent him into a smoke-wreathed tankslapper, spin and slide that ended close to the tyre wall.

The ensuing red flag, and another immediately after the restart when Marcus Ericsson crashed at Pouhon, helped keep Rosberg at the top of the times, but the post-session conversation led on the tyre failure, which television replays suggested had started much earlier in the lap.

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"We don't know [the cause] yet and it would not make sense to speculate about it," Rosberg commented, "I didn't feel anything before [the blow-out], so there was no warning - it was quite a shock in the first moment because I just didn't expect it and, suddenly, at 306km/h or 190mph to just lose control completely because something failed at the right rear is not good. There is a lot of damage to the car as well but, luckily, I did not hit the wall or anything."

The German, whose wife is expecting their first child in the next few days, admitted that he had played little part in minimising the outcome of the incident, from which he emerged to walk back to the pits.

"It wasn't down to my skill," he conceded, "It was just luck that I stayed away from the wall. But that was good."

With analysis already beginning both at Pirelli and back at Mercedes' base in Brackley, there will be speculation over the tyre choice for this weekend, and how the teams are using the tyres provided, although unexpectedly hot conditions may also have played a part in the incident, which remains an isolated one.

"Of course there is going to be a big analysis and [team-mate] Lewis [Hamilton] didn't drive because we don't know what [the cause] is," Rosberg continued, "It was too risky for him to go out again, so we need to look into it and maybe make some changes. But let's see."