Record-breaking multiple world champion Michael Schumacher's off-colour form since sensationally returning to F1 this season is 'a problem', Mercedes Grand Prix team principal Ross Brawn has acknowledged - and, what's worse, one that is 'difficult to understand'.

Brawn is the man under whose expert technical and strategic guidance Schumacher swept to all seven of his previous titles at the highest level between 1994 and his initial 'retirement' at the end of 2006, but having since elected to boldly make what was described in some quarters as the greatest comeback in sporting history in F1 2010, the formerly dynamic and unbeatable duo have been unable to recreate either that ultra-effective chemistry or on-track success.

The driver with a staggering 91 grand prix victories and 154 podium finishes to his name from his 'first career' in the top flight is one of only two competitors in the field this season to have been out-qualified by his team-mate in all four races. Indeed, in Shanghai last weekend - in changeable and tricky conditions that were once so famously his fort? - the German lagged behind younger compatriot Nico Rosberg to the tune of more than seven tenths of a second on Saturday afternoon.

On race day in the Chinese Grand Prix, 'Schumi' fared scarcely any better, trailing in the best part of a minute behind Rosberg, who ascended the bottom step of the rostrum for the second time this year as the Kerpen native could manage no better than tenth, slipping backwards and uncharacteristically making the wrong calls as his far less experienced countryman got it spot-on. Worse still, there was also more than a second between the pair's fastest lap times.

Amidst fears that Schumacher's advancing years have caught up with him and suggestions that perhaps he should cut his losses sooner rather than later by curtailing his comeback in order to ensure that no further unnecessary damage is done to his once-glittering reputation, Mercedes-Benz Motorsport Vice-President Norbert Haug has revealed that a new W01 chassis will be taken to the Spanish Grand Prix in Barcelona in just over a fortnight's time in an effort to resolve the 41-year-old's difficulties, with some sort of design flaw mooted as a possible explanation [see separate story - click here].

Brawn, meanwhile, has admitted that he is simply perplexed by the situation, and confessed to German publication Auto Motor und Sport that Schumacher's 'pace is a problem', with Rosberg having thus far notched up 50 of the Brackley-based outfit's 60 points in 2010. Had both drivers scored as highly, Mercedes would currently be vying with McLaren for the honour of leading the chase for the constructors' crown - rather than mired as the lowest of the contenders for glory down in fourth.

"Michael's problems are difficult to understand," revealed the Englishman. "It's not the difficult corners, but the technically simple ones [where he is struggling]. Until now he has been getting closer and closer to Nico, so what happened in China was totally against the trend."