Victory at Old Trafford would send the champions back to the top of the table with just two games remaining of one of the most topsy-turvy top-flight seasons in living memory.
Closing the gap: Ancelotti celebrates after the win against Tottenham took the Blues to within touching distance of leaders United
The Blues were in danger of failing to finish in the top four at the end of February and were 15 points behind and 1-0 down to United at Stamford Bridge on March 1.
But their fightback to win that game sparked a run that has seen them take 25 of the last 27 points on offer, slowly reeling in Sir Alex Ferguson's men.
United nevertheless remained overwhelming favourites to depose Chelsea as champions until this weekend, when the most dramatic of matches for both sides blew the title race wide open.
Asked what the key was to winning next Sunday, Ancelotti said: 'The moment. You need to use character and personality to play this match.
'Both teams have that, but it is the momentum.'
London calling: Ramsey (left) strikes the winner for Arsenal against United much to their London rivals' joy
The force certainly seems to be with Ancelotti's men, who beat Tottenham 2-1 yesterday thanks to two goals that should have never stood before watching United lose 1-0 at Arsenal after they were denied what appeared a clear penalty.
Sunday's game is nevertheless too close to call, with Chelsea having won at Old Trafford last season in what proved a title decider as well as coming out on top at Stamford Bridge in March.
They have since lost home and away to United in the Champions League, although they are arguably in better form now than they were at the start of last month.
Blues brothers: Ancelotti is calling on the likes of Lampard, Torres and Drogba (from left to right) to lead their revivial
Ancelotti, who revealed his players 'spirit' had also improved since their European exit, said: 'It is the best moment to play there because now our condition is good and the momentum is also.
'But obviously it will not be easy to play there.
'I don't think we are better (than United). I think we have the same power, the same strength, the same ability.'
Incredibly, retaining the title might not be enough for Ancelotti to keep his job, so he was understandably coy about debating if it would be the most satisfying trophy he had ever won.
'Football is strange.
'If it happens, it will be really strange.'
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