Sir Alex Ferguson has admitted winning the title this season will be harder than any other of his championship successes.
United go into the New Year six points clear of Chelsea at the top of the table and are favourites to lift the title in May.
Ferguson, who celebrates his 65th birthday on New Year's Eve, is refusing to take anything for granted and knows Chelsea still offer a great threat to his side's hopes.
"The influence of Chelsea and the big players they have been able to buy has made winning the league harder, there is no doubt about it," said Ferguson.
"It has not been easy for us because they have won the league quite comfortably in the last two years. That is why we wanted to make a good start this season.
"We have achieved that now. Hopefully, we can cash in on it."
Ferguson has also challenged his side to maintain their consistency to keep up the pressure on faltering Chelsea.
"They have not been tested like that before but if we can maintain our consistency, we will do it," added Ferguson.
Ferguson also hailed the contribution of Cristiano Ronaldo this season describing the Portuguese winger as the best player in the Premiership.
"We think Cristiano is the best in this league and have done so for a while," said Ferguson.
"The important thing for him is to be accepted.
"They still boo him away from home, like they did at Aston Villa last week.
"But those same fans are petrified whenever he gets the ball."
Ferguson also revealed that there is no problem in the relationship between Ronaldo and Wayne Rooney following all the talk over their World Cup bust-up.
"There is no battle between the two of them, they are the jokers in the dressing room and we have all been subject to their pranks," said Ferguson.
"I don't think Cristiano's form matters to Wayne because he has quite a laid-back, carefree attitude to life. He just gets on with it and enjoys himself."