BARCELONA, May - Sebastian Vettel blamed his relative slump to fourth in the 2013 Spanish Grand Prix on Red Bull's inability to manage their tyres as well as rivals Ferrari and Lotus.Vettel insisted he was happy with the potential speed and performance of his car: "I think we can be happy with fourth - or at least we have to be. The first three cars were a little too fast for us and in looking after their tyres they did a better job today."'WE NEED TO CATCH UP'He is confident there is no lack of raw pace in the Red Bull."If you talk about race distances, then it's a different game. I think the car is quick enough, but there's something we do to the tyres to make them wear more."Three people did better today so we need to catch up. We are not going to the pace of the car, we are going to the pace of the tyre," he said.Vettel added that Red Bull made a strategic mistake by initially trying to pull off a three-stop plan before resorting to four."I think we tried to hang on to the three-stop for too long in the race and had to admit towards the end we wouldn't make it," he said.Mercedes pole-sitter Nico Rosberg meanwhile admitted he was shocked at how quickly he slipped away from the front of the field, eventually having to settle for sixth.Team mate Lewis Hamilton, suffered a similar fate - finishing 12th after sharing the front row.Rosberg's drop down the order began on lap 12, when race winner Fernando Alonso passed him into Turn One in his Ferrari and then he was further demoted by Vettel and Kimi Raikkonen of Lotus.'THERE'S NO EXPLANATION'Rosberg, who finished ninth after starting from pole in Bahrain in April 2013, said: "I didn't expect it to be that tough. 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 both 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 added. "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"