City's 1-0 win over reigning champions and title rivals Manchester United in a tense derby at Eastlands on Monday saw them go top of the table on goal difference from their cross-town foes with two games remaining.
But Newcastle, pushing for a Champions League place, once more demonstrated their quality by winning 2-0 away to Chelsea on Wednesday, with Senegal striker Papiss Demba Cisse scoring twice to make it 13 goals in 12 games for the Magpies since his January transfer window move from German club Freiburg.
"Newcastle will be our hardest game -- harder than United," Mancini insisted. "It is strange but it is like this.
"They have had a fantastic season and are playing to get into the Champions League," the Italian added. "Their manager is one of the best managers here and it will be a tough game.
"It is another Champions League final on Sunday. We have two -- Newcastle and QPR," said Mancini, aiming to see City crowned champions of England for the first time in 44 years.
"We have to keep a big concentration until May 13."
Mancini maintains United, with games against Swansea and Sunderland, remain the favourites to retain their title.
Despite seeing City lose to both of those sides this season, Mancini has insisted Sir Alex Ferguson's men now face "two easy games".
Mancini said: "We have done nothing yet. We are at the top but we need to win two games.
"We need to play well and we need to score if we want to win. They (United) will get six points.
"We are happy for the crowd but it is important our supporters do not think it is finished. We need to be very calm.
"We need to concentrate on the next game, work well, recover.
"The message is to keep calm, like one and two weeks ago. Nothing has changed."
City's hopes of winning a first English title since 1968 appeared all but finished just over three weeks ago when they fell eight points behind United after a defeat at Arsenal.
But since that loss they have won four successive league matches and United a mere one in four.
City's revival has been helped by the return of controversial Argentina striker Carlos Tevez who spent several months in self-imposed exile in his homeland after a highly-publicised falling-out with Mancini following his apparent refusal to come off the bench at Bayern Munich in September.
Former United forward Tevez was quoted as saying on Monday he would like to stay at City but Mancini has insisted any contract negotiations can wait until after the end of the season.
Asked if Tevez could stay at City, Mancini said: "Why not? He has a contract, he is a good player.
"But now our concentration should be on Newcastle."