Tire worries take gloss off Mercedes' qualifying dominance

(CNN) -- If qualifying results were an indicator of race success this season, Mercedes would be dominating Formula One.

However, Nico Rosberg and Lewis Hamilton are both downplaying their chances of victory in Sunday's Spanish Grand Prix despite locking out the front row and earning the team's third successive pole position.

Only one race has been won by the pole-sitter out of four GPs this year -- world champion Sebastian Vettel at the second event in Malaysia -- as tire management has taken precedence over lap speed.

F1's official supplier Pirelli has modified its rubber compounds for this weekend to allay some fears over tire degradation, but Rosberg was left uncertain of his race prospects after heading off Hamilton by more than two-tenths of a second with a time of one minute 20.824 on Saturday.

"We have to be cautious after what happened in Bahrain," said the German, who was ninth last month after heading the grid.

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"Everyone has been working really hard to understand the problems and to understand how to get the most out of the tires as they are hugely complex.

"We were behind in Bahrain, but hopefully we have caught up now. I'm a little more confident for tomorrow. But the race is going to be a whole different thing given the challenge of the tires."

Hamilton, who was third in China after starting from pole, and fifth in Bahrain, paid tribute to the team's efforts to improve the cars.

"It's a real bonus for us to be so high up, and that's all down to the hard work that the team have put into this car, I can't thank them enough," said the 2008 world champion.

"I've not been totally happy with the balance of my car all weekend, even today, so we need to look at it this evening and see what we can do for the race.

"Limiting the degradation is going to be my major challenge as the general pace isn't too bad. It's going to be tough to keep these positions tomorrow but we'll give it our best shot."

Read: Vettel edges Alonso in practice

Vettel, who tops the championship standings by 10 points, qualified fourth ahead of Lotus' Kimi Raikkonen and the Ferraris of Fernando Alonso and Felipe Massa.

"I was pretty happy with the last sector, as historically it's been a bit weak for me," said Vettel.

"Now finally, after the last couple of years, I understand the last few corners a little better. We took a slightly different approach to some others using option tires only -- whether that helps us tomorrow we'll have to wait and see, but it will be an interesting race and a lot will be about tire degradation."

Massa was later handed a three-place grid penalty after being accused of impeding Mark Webber in the second Red Bull during Q2, with the Australian qualifying eighth behind Lotus' Romain Grosjean.

Those two moved up a place, as did ninth-fastest Sergio Perez -- whose senior McLaren teammate Jenson Button missed the third session and will start the race in 14th.

Force India's Paul di Resta qualified 10th ahead of Toro Rosso duo Daniel Ricciardo and Jean-Eric Vergne, while his teammate Adrian Sutil was 13th.

Williams' Pastor Maldonado, who won last year's race for his maiden F1 victory, was back in 18th behind his rookie teammate Valtteri Bottas.

They were both bumped up the grid after 16th-placed Esteban Gutierrez of Sauber was dropped three spots for impeding Raikkonen during Q1.