Spanish Grand Prix polesitter Nico Rosberg has admitted he was shocked at how quickly his Mercedes tumbled down the order after his first pit stop.
Rosberg led the opening stint of the race from pole, but his drop down the order began on lap 12, when Fernando Alonso passed him into Turn 1. He was further demoted by Sebastian Vettel and Kimi Raikkonen soon after.
The tire degradation of his Mercedes was such that he dropped to sixth place by the finish, just holding off Paul di Resta's Force India in the closing laps.
"I didn't expect it to be that tough," said Rosberg. "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 optimistic after his opening stint. "I thought, 'Who knows, maybe I can keep them behind all day?'" he added. "I knew they were quicker but I was quite comfortable and doing what I wanted to do. But then I just went backward so badly. It was unbelievable.
"It's so strange, so inexplicable. I'm working with my setup just towards the race, not even for qualifying, and we're so fast on one lap and then in the race so slow."
Lewis Hamilton in the second Mercedes, finishing a lapped 12th.