NICO Rosberg hailed "a good day at the office" as he delivered on his promise to bring his best form to the Spanish Grand Prix.

The Mercedes driver took his first pole position of the season after shining in qualifying.

Rosberg has been well beaten by team-mate and reigning world champion Lewis Hamilton in the opening four races - with the Briton, who has started on pole in all of them, downbeat after the session.

For Rosberg, his last pole came in the final round of last season in Abu Dhabi as he took 11 pole positions over the course of the year. This was his first of 2015 and the German admits it came at the perfect location.

"It just feels good because it's a great day in the office," he said. "It's not about past races or anything. It just feels great to be on pole, on this track it's particularly important because it's nearly impossible to overtake.

"[Qualifying in] China, for example, was also a good one, so it's not the first one. But today, yes, was a positive day and things went to plan."

Hamilton holds a 27-point lead at the top of the championship after his blistering start to the year but knows, on a track where overtaking opportunities are at a premium, his race could be decided as soon as the lights go out.

"It's always one of the opportunities," he said when asked about getting the start right. "You have that and some of the pit-stops. It's very, very difficult to overtake I think, as it's [been] shown and proved over the years.

"It is not very easy to dive down the outside at the circuit - it is really about getting down the inside off the line. But we have the exact same clutches so naturally we should have the same start."

Issues with the balance of the rear end of his car put paid to a pole bid for Hamilton - but he insisted he would not settle for second despite his healthy lead.

"I don't think so," he replied when asked if he planned a more calculated approach. "I think I'll drive the same as I did last year, it seemed to do me quite well. I generally didn't have the pace today.

"Nico did a great job and generally I don't know if I really had the balance where I like it. I did my best with it and I think tomorrow there's still a lot to play for."

Mercedes motorsport chief Toto Wolff was full of praise for Rosberg - who has come in for flak for his performances relative to Hamilton. "That was a brilliant lap from Nico to take his first pole of the season," he said. "It just goes to show that things turn quickly in Formula One.

"There have been lots of people asking questions about whether he could bounce back or had lost his momentum but they have to remember that these guys are tough, they're resilient and they're not so much affected by these ups and downs in results as people think."

Sebastian Vettel was again the best of the rest behind the Mercedes pair but the four-time world champion knows he faces a tough battle in getting his Ferrari anywhere near first place today.

"It's not a great place to be behind Nico and Lewis," he conceded. "I think it's the maximum we could do today, so I'm happy with qualifying.

"I think the weekend has been a bit rougher compared to the last couple in terms of finding the balance and the feeling with the car. We'll see what we can do."