Current Formula 1 World Championship leader Jenson Button admitted that his opening day of preparations for this weekend's Spanish Grand Prix had not got off to the most ideal start - a worrying statement indeed for the British star's rivals, given that in the morning running he had been almost half a second clear of anyone else on-track.

Barcelona marks the beginning of the long mid-season European leg of the F1 campaign, with Button having triumphed in three of the opening four 'flyaway' races to arrive at the Circuit de Catalunya with a comfortable twelve-point margin over his nearest pursuer - Brawn GP team-mate Rubens Barrichello - in the drivers' title standings.

Indeed, the 29-year-old has yet to finish off the podium in 2009, and on a warm and sunny day in the Catalan capital he put Brawn's new aerodynamic upgrades to the test, easily leading the way in FP1 but losing three tenths of a second later on in FP2 to be classified fourth-fastest overall, just over two tenths shy of the quickest time of the day, set by Williams ace Nico Rosberg.

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Though many have predicted the true acid test for the ex-Honda F1 outfit will come this weekend as the big guns unveil their updates in a bid to fight back, Button and Brawn's run of supremacy shows little sign of fading just yet - grip issues or no grip issues.

"The balance of my car hasn't been quite right today with our new aero upgrade fitted," the Frome-born ace confessed. "We have been struggling for grip which was a little frustrating, but we believe that we have identified the reasons. The good thing is that Rubens has collected some good data, so we can look at that overnight and have a more positive day tomorrow."

Indeed, Barrichello lapped faster than his team-mate in the afternoon session and ended up backing the sister car up ably in fifth place at the close of play, barely four hundredths of a second further in arrears. The veteran Brazilian has yet to get the better of Button in a race situation so far in 2009 - and if he is to avoid being swiftly cast into a supporting role, he knows he needs to rapidly start racking up the results.

"We have had a good first day," reported the S?o Paulista, "and focused our attentions on evaluating the new aero package that we have for this race. I ran the current package this morning for baseline data before fitting the new developments. My initial feedback is positive, and the car felt good and well-balanced. We ran a fairly conservative programme, which has given us valuable data to study overnight."

"It is always good to return to Europe after a hectic start to the season," summarised the Brackley-based concern's team principal Ross Brawn, "and particularly following the success of our first four races. Throughout that period we have been working hard on our first upgrade package of the season for the BGP 001, and it is very satisfying to see those developments on-track today.

"It has been a mixed day for us; Rubens was very happy with the performance, however we have experienced some issues with the performance of the package on Jenson's car. We are investigating the cause at the moment and have plenty of data to study overnight to resolve it."