Arsenal may not have won the Champions League tie but they approach the return with a serenity that is no longer within Barcelona's reach. The visitors lost a lead and then the match. In that run of events an initially effortless side became careless and vulnerable. They ought to reproach themselves and there is now penance enough in a return at the Camp Nou that will be stressful for Barcelona no matter how it is eventually resolved.

For the time being Arsène Wenger can settle for feeling proud of the maturity of Arsenal players whose temperaments did not shrivel even when Barcelona were at their most intimidatingly fluent. Instead Arsenal persevered in looking for weaknesses and eventually located them. Robin van Persie tied the score in the 78th minute with a drive from a tight angle on the left that ought never to have beaten Víctor Valdés at his near post.

There was further proof to come of Barcelona fallibility. Five minutes later Cesc Fábregas set Samir Nasri moving on the right and the cut-back was swept home perfectly by the substitute Andrey Arshavin. Barcelona had come as close as seems feasible to resembling normal, intermittently accident-prone human beings. Chances were squandered as if it could be taken for granted that there would be many more to come for Lionel Messi and others.

That assumption turned out to be a delusion and now there are matters of bare fact to occupy them. The centre-half Gerard Piqué was booked and will be suspended for the second leg. The Barcelona manager, Pep Guardiola, must therefore hope that the captain Carles Puyol's recovery from a knee injury does not falter between now and 8 March.

There was proof in abundance of Barcelona's technique and imagination, yet they were not ruthless in taking the full benefit. They had been 2-0 ahead here a year ago before drawing the first leg of the quarter-final but went on to rout Arsenal 4-1 in the return. That may account for the complacency when Barcelona neglected to drive home the advantage of David Villa's opener last night.

Arsenal's emergence as the single likely challenger to Manchester United for the Premier League title ought to have told Barcelona that there is more of an edge to Wenger's men nowadays. With an away goal recorded in a narrow defeat Guardiola's side deserve to remain favourites for the quarter-finals but that assertion is open to more debate now than anyone anticipated.

Meeting Barcelona is a predicament for virtually all teams but Arsenal's case is unusual. Rather than being the normal contrast of styles, this encounter saw Guardiola's side taking on opponents who would like nothing better than to emulate their methods.

Arsenal withstood that comparison. Even when the visitors took the lead through Villa in the 26th minute it came in a spell where they might just as easily have fallen behind. Neither team was particularly convincing when seeking to thwart the other. Their heart is not in such work.

That openness made for a frenetic yet also adroit spectacle. Barcelona could well have had a lovingly crafted opener after a quarter of an hour. Andrés Iniesta picked out Messi and he laid the ball back to Villa before breaking free on the return. It may only have been the wish to take the goal with a flourish that undermined him as a dinked shot went beyond the far post.

Arsenal were at least as effective then in unpeeling a back four. Following good work by Theo Walcott, Fábregas lobbed a pass to Van Persie, who rattled a shot that was saved by Valdés. Even if Wenger's men blundered in their effort to spring the offside trap, Messi sending Villa in for the opener, Arsenal had been trying to dictate the terms of the match and so break a Barcelona rhythm that is as natural as a pulse.

There had been indications from early moments that Barcelona are less sure in defence without Puyol. In that aspect the visitors could be seen as fallible human beings. They were not in full command when a run like Walcott's unpeeled the right side of the defence in the 55th minute, although his low ball was cleared. There was enough disquiet in Barcelona for Piqué to receive his yellow card, after bringing down Fábregas.

There was a gathering conviction to Arsenal and, in consequence, the odd indication of alarm in the visitors as the second half assumed its shape. It took 20 minutes or more before Barcelona enjoyed a studied period of possession as they probed around the edges of the Arsenal penalty area. Two substitutions reflected managerial outlooks.

Guardiola's preference was conservative, as Seydou Keita took over from Villa, while Wenger pitched on the creative Arshavin for the holding midfielder Alex Song. The match had turned pensive. That was natural after two sides who had regularly swept around this pitch realised that they resemble normal footballers in one respect. They, too, can get very tired. Arsenal still mustered a perseverance that took them to victory.