Belgian F1 GP: Nico Rosberg not reassured over tyres after blowout

Rosberg was pitched into a high-speed spin on the approach to Blanchimont in Friday afternoon F1 practice at Spa when a rear tyre burst.

After a detailed investigation, Pirelli concluded that it was the result of an external object - such as a piece of debris - cutting the tyre rather than any form of structural failure.

But after qualifying second to team-mate Hamilton, Rosberg said he was not completely reassured.

"The problem is that we don't really understand it," he said.

"There are theories but there is no real evidence, so that's a bit worrying for sure.

"We're keeping a very close eye on it and we've done some measures to make sure it doesn't happen again.

"We'll just do the best we can and hope it all stays together."

Hamilton added he was unsure what would happen with tyre wear, regardless of any potential for a repeat of Rosberg's drama.

"The tyres are an unknown from a degradation point of view, but also we've seen Nico's issue yesterday, who knows what will happen in the race?" he said.

HAMILTON: MIDDLE SECTOR CONQUERED

World champion Hamilton clinched the FIA's 2015 pole trophy in Spa qualifying as he claimed his 10th pole in 11 grands prix.

Although it was his fourth pole at Spa, he said it was the first time he had truly mastered the middle sector - where the majority of his 0.450-second margin over Rosberg was established.

"Sector two has been probably in the past a bit of a weak point," Hamilton admitted.

"I knew the lines but could never really put the corners together.

"Definitely on those last two laps that was a very, very strong area for me."

Rosberg had the edge on Hamilton during Friday practice but conceded he had no answer for his team-mate in qualifying.

"I lost a little bit this morning in free practice, we went the wrong way," said Rosberg.

"Then it came back really nicely and I had a good balance in qualifying.

"Lewis was just too quick in the end. He found quite a lot extra which I didn't have in my pocket."