LONDON, England - Nico Rosberg has tipped Mercedes' rivals to have closed the gap during the three-week break since the April 2014 Chinese Formula 1 GP.The UK-based team has utterly dominated the 2014's opening four GP's of the new turbo V6 era but the start of the European season is traditionally the scene of vast improvements up and down the pits lane as teams add the first major developments to their fledgling cars.Driver Nico Rosberg told SpeedWeek: "Until now we'v had a big gap to our nearest competitors but three weeks have passed since China and for sure a lot has happened.ON GUARD"We will have to wait and see," said the German ahead of this weekend's Spanish GP. "I hope we still have an advantag but we will have to see how big it is. I'm confident we will still be fastest but we are also aware that the competition has been working around the clock to catch up."At the same time, Barcelona is usually a key indicator of how the rest of a F1 season will look. Not only is the Catalunya track a frequent test venue for the sport but the nature of its layout is a good indicator of how a car will perform on most other tracks.Paul Hembery, tyre supplier Pirelli's F1 boss, said: "A car that performs well in Barcelona should perform well everywhere." Stay with Wheels24 for the 2014 Spaninsh GP this weekend.