Manchester City are buckling under the weight of their immense ambitions. The lack of fluency against a bold and adventurous Swansea City led to defeat for Roberto Mancini's side. For the first time since 15 October a set of fixtures has been completed without them at the head of the Premier League. It is Manchester United who top the table by a point.

That is a slim margin, but it contributes to the sense that City have begun to seize up. Few opportunities were devised and they cannot count on luck to come to the rescue. An apparent equaliser by Micah Richards in the closing moments was denied correctly because he was offside by a fraction, but on another afternoon the officials' judgment might not have been so acute.

As it was, there was just one goal and it was relished by Brendan Rodgers's side, who merited their triumph. This match had always been a trial for the visitors. Mancini was sufficiently concerned to reconfigure his side at an early stage, with a visibly irritated Gareth Barry withdrawn so that Sergio Agüero could come on in the 38th minute and offer verve to a rejigged lineup.

The sight of Yaya Touré shooting high just before the interval was itself an improvement. City's difficulties were not so great a shock since Swansea began the day sharing the best defensive record at home in the Premier League with Liverpool, even if that stringency has been fading a little. The real surprise came with City conceding a penalty in only the second minute.

Perhaps that ought to have been interpreted as a first indication of alarms to come. There was contact between Wayne Routledge and Joe Hart, but the poor shot from Scott Sinclair was dealt with comfortably. The midfielder may have been put off by gesturing and apparent comments from the goalkeeper. Apart from that incident, the endeavour led to little excitement, despite the fact that City have scored more away goals than any other team in England's top flight.

The record held by a side who had come to the Liberty Stadium as leaders of the Premier League does not necessarily imply that all of City's verve is intact. On Thursday, for instance, they had gone down 1-0 in Lisbon to Sporting, making the return leg problematic in that Europa League tie. It was unsettling, too, that the centre-backs Joleon Lescott and Vincent Kompany were absent through injury.

Of course, all the concerns that City may have are liable to be ridiculed when so much is in the favour of a club with an apparently limitless budget. It is far too late now for the average observer to harbour much compassion for them, but the bid for significant trophies and the public assumption that City will prevail does put stress on the squad.

There is no resemblance to the period when it was almost being taken for granted that, say, David Silva had already clinched the footballer of the year award. Now we will discover if City can grind as well as glide. Nothing will come easy now that vulnerability has been laid bare.

Those who like a moral tone even in football could rejoice. Swansea, with comparatively small means, outdid City in open play for the majority of the afternoon. That penalty at the outset turned out not to be just a random incident. It highlighted insecurity that would be repeated as Swansea made inroads and looked as if they had never heard of the opposing players, let alone dreaded being pitted against them.

That indicated a startling development process. The hosts here are far more capable of posing problems than they were when going down 4-0 to City at Eastlands on the opening weekend of the Premier League campaign. Any concerns of late have been of a lesser order for Rodgers's lineup, but he might have been unsettled by the fact that they had not kept a clean sheet on their own pitch since a defeat of Fulham on 10 December.

That was put to rights. Swansea also achieved a first win at home since overcoming Arsenal in mid-January. Correcting that trend was no simple matter for Rodgers when Mancini seemingly had so much firepower, yet Swansea did thwart them by attacking when feasible and showing discipline when City were in possession.

Mancini's side looked frustrated and were indignant when no penalty was given after an hour, although Joe Allen's contact on Touré had occurred outside the 18-yard line. Later, Micah Richards would demand a save from Michel Vorm, but City were to be stifled by their own predictability and undone with a header from the unmarked substitute Luke Moore after 83 minutes.

The smothering of the free-spirited attacks that we saw not so very long ago was to be anticipated to an extent, but City ought still to breathe a little more easily than they did here. That, indeed, is Mancini's responsibility. He therefore has to commit himself to a new task. Improbable as it might seem, he has players to deal with who, despite the limitless millions, will feel apprehensive about the weeks to come.

Man of the match Steven Caulker (Swansea City)