Manchester United boss Sir Alex Ferguson says Chelsea are their biggest rivals this season.

United visit Stamford Bridge on Sunday looking to close the two-point gap Carlo Ancelotti's team have opened on the champions.

"It's looking that way," he told Inside United. "It seems they will be the ones to get in front of."

However, after tweaking their tactics to get the best out of Didier Drogba and Nicolas Anelka, Ancelotti has enjoyed an excellent start to Premier League life.

"It is a very experienced team," said Ferguson. "I said last year it is a team that can only improve results by the system they adopt.

"Ancelotti has brought in the system he used at AC Milan, with the diamond in the middle of the pitch.

"It is hard to break down when you get into their third because they have got bodies compacted into the central area of the pitch.

"Jose Mourinho played one up and five in the middle of the park - that also made Chelsea very difficult to beat, but in a different way to what Ancelotti is doing.

"Ancelotti's got the two strikers, and that is a major change compared to Luiz Felipe Scolari and Claudio Ranieri."