Roberto Mancini claims Manchester City's FA Cup defeat by Manchester United earlier this year was the key turning point in their challenge for the Premier League title.
City lead their great rivals on goal difference at the top of the table with two games left, the first of which is a difficult visit to high-flying Newcastle United on Sunday.
Mancini's team make the trip having beaten United at Eastlands to complete a rare derby double against the cross-city rivals they swept aside 6-1 at Old Trafford in October.
But it is the third-round FA Cup defeat by United, when captain Vincent Kompany was sent off in the early stages, that Mancini insists has instilled belief and resolve in the players challenging for City's first title since 1968.
"The important game was the FA Cup when we lost 3-2 at home in January - not when we won 6-1 there, that was an easy game," Mancini said.
"We lost 3-2 when we were down to 10 players after 10 minutes, but we recovered and had the chance to score the third goal.
"They didn't have a shot in the second half with 11 players. That game was important. It changed our mentality against them."
Mancini added that there was no need to give an inspirational talk in the dressing room before the recent derby victory game, which City needed to win to have a realistic chance of the title.
"Against United we knew it was the last chance for us," he revealed.
The City manager insists also that the game at Newcastle, who are aiming for a top four finish and Champions League qualification, could be even more demanding. He said: "This will be another tough game. Newcastle played a fantastic game against Chelsea (to win 2-0). They try to win every game, and they have a good mentality.
"It will be very difficult with the crowd and Newcastle are a good team. Our approach and our mentality will be very important, it will be a key game."
United face Swansea City at home, the easier task on paper, in a later kick-off which will see them already knowing the result from Tyneside.
Mancini, who has all his players fit, added: "We have the best team in Manchester this year, but to prove we're the best in the country we need to win the next two games.
"It's not important to beat United twice or to beat Chelsea, it's important we win the championship in the end. I'm very happy we have the chance to do that after being eight points behind United at one stage."
Newcastle strengthened their bid to clinch an unlikely fourth-place finish with a fully-deserved victory at Chelsea in midweek - their seventh win in eight games to cement their hold on fifth spot.
Back on home soil for the last time this season in front of a sell-out 52,000 crowd, they hope to go out on a high.
"We have a chance to qualify for the Champions League, and this is a very important match," Yohan Cabaye, the France midfielder, insisted.
"We're looking forward to the challenge against a very good Manchester City side. We've got two games left to go and we're in a great position, so we want to end the season on a high if we can."
Cheik Tiote, the Ivory Coast midfielder carried off at Stamford Bridge with a head injury that required seven stitches after a clash with John Obi Mikel, is expected to be fit to take his place.
"It's a huge game for both clubs, and certainly the biggest match for Newcastle in my time here. It's one I'm really looking forward to, and I'll be okay to play," he said.
Defender Danny Simpson is again likely to be sidelined with a thigh problem, meaning James Perch continues to deputise at right-back.