Schuey: Doubts over McLaren tyre swap

While McLaren gets to grips with its new rubber, the four-time world champ says the Williams duo of younger brother Ralf and Juan Pablo Montoya, who has been touted as a possible successor to Schumacher at the top of F1, will pose his biggest threat.

"Obviously there is a little question mark concerning McLaren due to them changing tyre company - we don't know how quickly they can adapt to that," Schumacher told Reuters at Ferrari's annual ski trip in Italy. "But these two teams [Williams and McLaren] are the two big threats as usual, except that Williams is in a much better position, for me, this year than they have ever been.

"Although they have put their own chances on a lower level, so as not to build up the pressure too much, I think they are going to be very strong and we are going to face a lot of difficulties from them."

Schumacher returned to testing this week for the first time since the final grand prix of last year in Japan. The German broke the lap record on his first day back behind the wheel of an F2001 at the Maranello team's Fiorano test track.

The world champion is confident he can retain his title this year, but while he is expecting some tough opposition from the Williams squad, he is intrigued as to who will come out on top at McLaren between Coulthard and his new hotshoe team mate Kimi Raikkonen.

"I don't know if he can or can or cannot [go faster this year]," added Schumacher. "David remains David - he is a strong racing driver and he is going to be a challenge to me no doubt. How much is it [McLaren] going to be David and how much is going to be Raikkonen is an interesting question to be answered."