City regained the initiative in their battle for the Barclays Premier League crown with arch-rivals Manchester United with victory in a tense derby on Monday.
Mancini's men now lead the table on goal difference with two games remaining but the Italian, who has continually played down City's chances over the past month, is wary of the high-flying Magpies.
He said: "Newcastle will be our hardest game - harder than United. It is strange but it is like this.
"They have had a fantastic season and are playing to get into the Champions League.
"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.
"We have to keep a big concentration until May 13."
Mancini maintains United, with games against Swansea and Sunderland to come, are the favourites to retain their title.
Despite having lost to both of those sides this season, Mancini has referred to United's run-in as "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 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 appeared to be out of contention as they fell eight points behind after a damaging defeat at Arsenal just over three weeks ago.
Since then they have won four in succession while United have won just one from four.
One of the most instrumental figures in City's late surge has been controversial striker Carlos Tevez.
Despite his well-publicised dispute with the club earlier this season, the Argentinian was quoted on the day of the derby saying he would like to stay at the Etihad Stadium.
Mancini has not ruled out that possibility but has reiterated it is a matter he intends to address in the summer.
When asked if Tevez staying was a possibility, Mancini said: "Why not? He has a contract, he is a good player.
"But now our concentration should be on Newcastle."