BARCELONA, Spain - Nico Rosberg and Lewis Hamilton were right to be cautious after their Saturday (May 11) Spanish Grand Prix high turned into another Sunday slump for Mercedes.The 2008 World champion Lewis Hamilton started on the front row of the grid and struggled to 12th place with four pitstops. WORST RACEHamilton said: "It's got to go down as one of the worst races I've had for a long, long time."Mercedes had celebrated their third pole position in a row on Saturday (May 11) during qualifying, with Rosberg taking the top slot for the second successive race, but with plenty of reservations about their race pace.The car has been stunningly quick over a single lap but there are no points for qualifying and problems with tyre degradation have prevented Mercedes from cashing in on race day afternoons.Rosberg, who went from pole to ninth in the 2013 Bahrain GP during April, ended up sixth in Spain.CONFUSEDHamilton added: "I really don't know what the problem is. I'm lost. We were slow and I had no grip for some reason. It was really tough, way too tough. I felt like I was going backwards, which I obviously did.""I pushed as hard as I could, saved as much as I could at the times I needed to. But the grip, the tyres, never came in, never arrived," he said.In a sign of how far he had slipped, Hamilton complained over the radio that he had been overtaken by the Williams of Venezuelan Pastor Maldonado.That would not have been a drama in 2012, given that Maldonado won the Barcelona race from pole, but the struggling former World champions have not scored a point yet this season and Maldonado had started 17th and served a drive-through penalty.Mercedes motorsport head Toto Wolff, who is still a Williams shareholder and whose wife Susie is a Williams development driver might have winced at the words but he said he had been listening to a different feed.Wolff said: "I had physical pain because I knew how he felt in there."The result meant Hamilton slipped from third overall to fourth, 39 points behind Red Bull's triple World champion Sebastian Vettel, while Rosberg is ninth in the standings, a further 28 points behind.The race was won by Ferrari's Fernando Alonso, with Kimi Raikkonen second for Lotus.Team mate Rosberg said: "We didn't have the pace of the quickest cars today and there's a growing gap to three teams ahead of us which we need to get on top of."QUICK FIX NEEDED"We have a very quick car but we have to make it work for us in the race," he said.Austrian Niki Lauda, the team's non-executive chairman, said they had to find a solution and find it fast.Lauda said: "The car is quick, there's no question about it. But the tyre consumption...look at Vettel, the same problem. He couldn't get anywhere near the Ferraris and Raikkonen.""So, this is a problem which we need to fix but I don't know how. They have to fix it. No question," he added.