It was the 100th goal City have scored at home under Manuel Pellegrini, and very few have been quite as important. A Chelsea win would have seen them move eight points clear of City after just five matches—hardly insurmountable given the number of games left to play, but an uncomfortable position to be in so early in the season. As it is, City are now five points behind the leaders after an undoubtedly tougher start to the season than any of their rivals.

The title race is gathering momentum already, and with their hard-fought result, City remain in the hunt. Many may have felt Pellegrini would urge his side to be cautious given Chelsea’s strong start to the season, but his commitment to attacking football is strong. A two-man forward line of Edin Dzeko and Sergio Aguero, with David Silva moved back out to the left, poured scorn on the idea he may change tact and play three in midfield. It seems they won’t change their way of playing for anyone.
So far this campaign, we've seen a new Chelsea, one buoyed by a summer transfer window that saw them strengthen in key areas, ushering in a renewed belief that they can win the league for the first time since 2010 and encouraging a more dynamic, attacking style of play. Where in the past they've been accused of "safety first," this season the handbrake has been released and the goals have flowed.
However, Jose Mourinho is known for his pragmatism, and coming to the home of the champions requires certain concessions. They went with three in midfield and looked to stifle City's play, waiting for a chance to strike on the counter.

And that's exactly what happened when Andre Schurrle finished off a wonderful break moments after City had gone down to 10 men following the dismissal of Pablo Zabaleta. Sensing this was their time to punish City, Chelsea broke with pace and dynamism after defending a corner, and in the blink of an eye, the German had scored from close range after Eden Hazard's ball across the box.
This, it seemed, was typical Mourinho: soak up pressure before striking at the key moment.
Zabaleta can consider himself unlucky to see red, the victim of a fussy refereeing performance from Mike Dean, who remarkably managed to book six players in a first half that rarely seemed malicious. He left himself little room to manoeuvre, and when Zabaleta and Diego Costa clashed just after the hour mark, the Argentine was dismissed.

But City weren't finished. They pressed hard for an equaliser despite their numerical disadvantage and found a breakthrough with five minutes remaining. James Milner, who was superb throughout and operated in at least three different positions during the game, slipped the ball to Silva, who played a wonderful ball back into the England man's path. Milner then cushioned the ball towards Lampard, who hit a first-time volley past the impressive Thibaut Courtois.

In an age where muted celebrations have exasperatingly become the norm, Lampard can, of course, be forgiven for his. He is so ingrained in Chelsea's history that many are finding it difficult to acknowledge he's a City player. He scored 211 in his time at Stamford Bridge—plenty of them important ones—and he clearly struggled with scoring his first for City past his old teammates.
One other huge positive for the home side was Eliaquim Mangala's debut. Pellegrini has so far kept the Frenchman wrapped in cotton wool, but he was unleashed for a crucial game and performed superbly. He looked incredibly quick and strong, with a leap that suggests an aerial dominance few other Premier League defenders can match.

There was an embrace between Pellegrini and Mourinho at the end which hinted at a mutual respect after a hard-fought 90 minutes, but the spiky nature of their post-match press conferences soon saw them revert to type.
Pellegrini appeared unhappy with Mourinho's regressive tactics, referring to Chelsea as a "small team" after the match:

I don’t know if both are happy. For me, I am not satisfied with the result. I think during 90 minutes we played against a small team trying to defend, trying to keep 10 players in front of their goal and a team that wanted to win from the beginning.
We had two or three clear chances to score at the start of the second half, and after with 10 players, we continue we trying to win. I am satisfied with the performance of my team, but I am not satisfied with the score.

They don't like one another—that much is clear—and they look set to go head-to-head once more this season (January 31), with their respective teams looking the strongest title contenders.
It's a feud which looks set to continue as the season progresses. City kept their rivals within reach with Sunday's result, but it's Chelsea with the early advantage.