Michael Schumacher admits that his return to the F1 grid has been more difficult than he expected as he prepares for the final four races of the year.

The seven-time champion has finished inside the top six on only four occasions having come out of retirement to join Mercedes at the start of the season - a series of results which have led to questions about whether or not he would elect to continue into the 2011 campaign.

That talk has been played down by both Mercedes and Schumacher himself and the German admitted that there was work to be done in 2011 to allow the team to fight back from a disappointing 2010 season.

"It has been a much tougher year than we expected," he said. "If you think of the performance that the team was able to do last year, expectations were high. We have not been able to fulfil those expectations. At the same time it has been a long-term project and if I look back at how long it took with Benetton and with Ferrari to build up a team and then finally to take success it has never been possible to do that in the short term.

"The nature of the fact that the team used to be a big team, such as the top three teams, then was reduced to a much smaller team during last year due to circumstances that everybody knows, we are now a rather small team compared to the top running teams. That, in the situation that we are right now, makes it, naturally, a little bit more difficult. But then we have made decisions and steps to get back to the winning route although naturally it will take time."

With his own car out of contention for victory this weekend in Japan - barring a remarkable set of circumstances - Schumacher admitted that the Red Bulls of Mark Webber and Sebastian Vettel would be difficult to beat at a circuit where a good car is vital to securing the best result.

"In a way, it is a high challenge track, and drivers, yes indeed, can give a great input on this kind of track, especially in the first sector, but nevertheless, the car is mega-important because of this first sector," he said. "If the response from the front end in particular, with all these longish corners, is weak, you suffer quite a lot and in this respect, looking at the nature of the Red Bull car, I think it's going to be very strong in my view, but then I know that McLaren is pushing very hard on developments, so we will see whether they can keep up or not. That's going to be a tough one."