German cannot explain why his Mercedes was not able to maintain grip at Circuit de Catalunya, calling the experience 'so strange, so inexplicable'.
Nico Rosberg cannot get to grips with tyre-wear issue at Spanish Grand Prix
The one consolation for Nico Rosberg and Mercedes-GP was that it had not been as bad as Bahrain, where on the Sakhir International Circuit, the German had started from pole position and fallen back through the pack to finish in ninth place as his car's well-publicised problems at making the fragile Pirelli tyres last struck.
Starting from the first row on Sunday in Spain, not only did he last at the front for longer then he had in Sakhir three weeks ago, when he lost the lead within three laps, he managed to finish in sixth place after holding off a challenge from Paul di Resta's Force India in the closing laps.
An improvement, but the frustration for Rosberg was the same, as he was left baffled at how he had fallen down the order so quickly once he had been passed for the lead by eventual race winner Fernando Alonso on lap 12 of the 66-lap race.
"I didn't expect it to be that tough," Rosberg told Autosport. "I thought we would be in a better position. It was really difficult out there, but why? Why on one lap are we so fast and on a long run so slow? It's strange, and it's not just one axle, it's front and rear.
"Both are struggling and there's no explanation."
Rosberg admitted he was confident after his opening stint that he could hold on at the front of the field, but was shocked when he could not.
"After the first stint, I thought 'Who knows, maybe I can keep them behind all day'?," he said. "I knew they were quicker, but I was quite comfortable and doing what I wanted to do. But then, I just went backwards so badly. It was unbelievable.
"It's so strange, so inexplicable. I'm working with my set-up just towards the race, not even for qualifying, and we're so fast on one lap and then in the race so slow."
Another bonus for Rosberg was that for the first time this season, he finished ahead of teammate Lewis Hamilton.
Hamilton suffered from similar problems to Rosberg, but on a bigger scale, with the Briton, who had started second, eventually finishing in 12th position, the first time in 2013 he had finished outside the points.
"It was an experience that I don't really want to go through again," the 2008 world champion told Autosport.
"Today, I was doing absolutely everything the same as I did in Bahrain. But I had no grip, I couldn't push, and if I did, the tyre just went off immediately.
"I'm absolutely lost today. I don't know what went wrong."
Hamilton had hoped Mercedes had found a step forward in the three weeks since Bahrain, but acknowledged more still had to be done.
"Even though we brought upgrades, we didn't make any progress with the degradation," he said.
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