By Simon Stone, Press Association Sport Chief Football Writer

Sir Alex Ferguson has tipped Chelsea to be the main threat to Manchester United's dream of completing an historic title four-timer.

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

And, although Arsenal have also enjoyed a fine start to the campaign and Manchester City have squeezed into the top four after their summer spending splurge, Ferguson feels it will be a meeting between England's top two.

After going through four managers in two seasons, it was felt Ancelotti might have a tough job settling Chelsea, particularly as he has no previous experience of managing in England.

"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."