SPA - World championship leader Nico Rosberg turned the screw to gain another advantage over his rival and Mercedes team-mate Lewis Hamilton on Saturday when he secured his fourth straight pole for Sunday's Belgian Grand Prix. On a day of capricious and treacherous conditions at the majestic 7.004km Spa-Francorchamps circuit in the Ardennes forest, the 29-year-old German produced a meticulous performance that delivered an error-free and fastest lap to claim his seventh pole this year and the 11th of his career.He clocked a best lap of two minutes and 5.698 seconds to outpace Hamilton by two-tenths of a second. Behind the two Mercedes, defending four time champion Sebastian Vettel was third fastest for Red Bull ahead of two-time former champion Fernando Alonso of Ferrari and fifth placed Daniel Ricciardo, winner in Hungary a month ago, in the second Red Bull.Valtteri Bottas was sixth in the leading Williams ahead of Danish rookie Kevin Magnussen of McLaren, Kimi Raikkonen in the second Ferrari, Felipe Massa in the second Williams and Jenson Button in the second McLaren. Hamilton struggled with his brakes and handling in the slippery conditions, but will start alongside Rosberg on the front row as he seeks to trim his 11 points lead in the drivers' championship with eight races remaining.It was the Mercedes team's first front row lockout since the Canadian Grand Prix in June, five races previously. For Rosberg, it also brought his first top three qualifying result in Belgium in eight attempts as he seeks to land his first podium finish at the venue. "It is awesome to achieve this at this track," said Rosberg. "This is one of the most special tracks of the year. It was not quite as difficult out there as some other occasions because the conditions were pretty much always consistent for intermediates."Asked where he felt he had found an advantage over Hamilton, he said: "In Eau Rouge, I was feeling good through it, really going for it -- and maybe that is where I made my time." Hamilton, who has not started from pole in seven races since the Spanish Grand Prix in May, controlled his disappointment behind a pair of sunglasses after the session."I had a glazed front left brake disc, so the car was pulling to the left or right and there was nothing I could do," he explained. "I couldn't get rid of it. I had to bring the brake balance further back, losing massive amounts out of Turn One. It's more fun in the dry, but it's still a huge challenge here, just as Nico said, and, considering that, it was quite good today." "It would have been nice to have finished further up the grid, but obviously the gap is quite big," said Vettel, who had missed most of Friday practice during which a new engine was fitted to his car. "It was the best we could have done today."Hamilton was not dismayed despite seeing Rosberg take his sixth pole in the seven races since he last took one. "This is a circuit where you need to have confidence in the brakes. You could see in Q3, when I was going straight on as the left brake wasn't working for some reason, I don't know why. It went into glazing. When it glazes, it is very hard to recover, you have to put the brake balance forward. It's not that easy to clear it."He added that he was confident the problem will be resolved and he will have a trouble-free race. "Over the evening, you can scrub it and get rid of the glazing. It will be okay for the race tomorrow. I am not too disappointed. If you look at previous years, P2 is the best place to start here. I started from pole here last year and got overtaken by (Sebastian) Vettel. I think it gives you the most opportunity at the start so I am happy to be up there."Rosberg was on top after Q1 when Pastor Maldonado of Lotus, who had a big spin at the chicane, and Nico Hulkenberg of Force India were among those eliminated. Missing the cut after Q2 were Daniil Kvyat, his Torro Rosso teammate Jean-Eric Vergne, Sergio Perez and his Sauber partner Adrian Sutil, Romain Grosjean of Lotus and Jules Bianchi for Marussia.