The conventional wisdom about Arsenal’s title push is that they have the easy run-in. Compared to Chelsea and Manchester United it is apparently a piece of cake. A home game against the team above the relegation zone by virtue of goal difference was supposed to be one of the tastiest of the lot, but Arsène Wenger’s team came oh-so-close to suffering a terrible dose of indigestion.

The game hinged on a critical incident a minute before half-time, when Thomas Vermaelen was shown a red card for tangling with Guillermo Franco. Not so easy all of a sudden. But the way Arsenal regrouped, resettled, and finished off West Ham showed they have the heart to take this adventure as far as they possibly can. Their title rivals may have games in hand, but Arsenal wake up this morning looking down on everyone.

The quest now moves to Birmingham. For different reasons recent games have all felt central to the plot of this unfolding drama, but St Andrew’s is a challenge that feels particularly pivotal in terms of Premier League aspirations. Against the kind of direct opponents they have been known to find unsettling, they will travel without their first-choice centre-halves. As a prelude to Barcelona, its importance cannot be overlooked.

As much as it was natural for Arsenal to be thankful for the return of their captain and top scorer Cesc Fábregas, the reinstatement of Alex Song after a two-match suspension was equally reassuring. The Cameroonian anchor has been crucial, and has matured into an important safety net in front of a back line with a tendency towards fragility.

Alongside him Denílson was chosen ahead of Abou Diaby. This was surprising. A more creative player is the norm alongside Fábregas and Song in midfield, but Denílson was favoured ahead of Diaby and Tomas Rosicky. Was this an experiment with Barcelona in mind?

Arsène Wenger was firm that West Ham was the absolute priority, and the Denílson selection turned out to be an inspired one. The Brazilian provided his team with the gift of an early goal. He was alert to the opportunity to pickpocket Valon Behrami and skillfully smuggled the ball to Nicklas Bendtner. When it was returned to him, Denílson drilled a first-time shot into the bottom corner.

Denílson has elicited his fair share of moans from the crowd this season, especially during a period when he found the going tough in midwinter. But here he sparked. Maybe he had borrowed some of Bendtner’s enormous supply of confidence (there is plenty to spare). This was his fourth league goal from 16 starts this season, and not for the first time it was an important goal, too.

He might have had another soon after. At the end of a tippy-tappy move, Denílson chested the ball down and volleyed goalwards. His flourishes were all the more valuable as Arsenal were not at their fluent best in the first half. Fábregas took an early kick on the foot and looked very unhappy with the perpetrator, Behrami, with whom he later had words. The Catalan was not running freely at all and strained to exert any great influence. Samir Nasri and Andrey Arshavin were a little flat, too.

West Ham had enough glimmers to suggest Arsenal would be foolish to take this at too much of a presumptuous stroll. Junior Stanislas broke down the right flank and whipped in an inviting cross which Mido couldn’t reach, then Gaël Clichy and Sol Campbell made excellent interceptions as West Ham built towards goal.

In the last minute of the half, the pendulum swung viciously. Franco surged onto a high pass and Vermaelen missed the header, then in his desperation to retrieve the situation was clumsy as he tussled with the Mexican. Although contact was minimal, Franco tumbled inside the penalty area. The referee Martin Atkinson was so far behind the play he was closer to the centre circle than the penalty box, but trusted the instincts of his linesman. Not only did he point to the spot, he sent off Vermaelen. Wenger was infuriated, and waited at half-time to remonstrate with the officials.

In the meantime, Diamanti struck his kick well, but Manuel Almunia plunged to produce an inspired save. His record with penalties is one of his best features, and Arsenal were immensely grateful.

Interestingly, Wenger chose not to make a substitution and Song dropped back to fill in at centre-half. He had his work cut out as West Ham set about the second half with attack in their hearts. It was not long before Arsenal did make a change, with Diaby replacing Bendtner and Arshavin leading the line.

Arsenal hauled themselves back again to force the game up towards the edge of Rob Green’s box. Emmanuel Eboué became increasingly influential, and his ability to win free-kicks kept up the pressure. Campbell ambled up for a corner but headed too close to the keeper.

Gianfranco Zola, so desperate for points, sent on the attacking power of Carlton Cole and experience of Benni McCarthy. With 12 minutes to go a sizzling left-footer from Cole shuddered against the base of Almunia’s far post.

Back came Arsenal, and Matthew Upson handled in the box inexplicably as Fábregas bore into the danger zone. Another penalty. In the swirling rain, the captain steeped up to rifle in, Green diving the wrong way. “We are top of the league,” sang the crowd giddily. It ain’t easy, but it sure is scintillating.

ArsenalWest Ham UnitedPremier LeagueAmy Lawrenceguardian.co.uk

West Ham, given their relegation fears, would normally have given thanks for a point against Chelsea, but they felt wronged here when Matthew Upson was deemed by the referee, Mike Dean, on the advice of his assistant, to have fouled the substitute Daniel Sturridge. Frank Lampard converted the penalty at the third attempt, after the official ruled out the first two kicks for encroachment.

Chelsea had met with stiff resistance from West Ham, who know they need to stop games from being quite so eventful if they are to survive in the Premier League. The side had been conceding at a rate of two goals to every match before this fixture. No one accused them of a reckless romanticism here and they threatened to hold on to the lead they had gained with a penalty of their own.

There were breaks on the flanks now and again, but for the most part Chelsea found 10 men barring their way. The visitors are accustomed to that sort of approach, but would not have anticipated the implacable manner in which West Ham applied themselves.

Any reputation for brittleness had receded. Not even the muscle injury that ended Danny Gabbidon’s involvement had immediate consequences as James Tomkins rejoined a line-up in which he had started the previous three games.

It had looked unlikely that West Ham would score, but a covering Ashley Cole fouled Jack Collison from behind as he collected a pass and Alessandro Diamanti sent Petr Cech the wrong way to convert the penalty in the 45th minute. Chelsea’s true difficulty, all the same, lay in a lack of invention.

They might well have had a penalty early in the game, when Gabbidon’s hand made contact with the ball, but it did look for a time as if they would be dependent on Didier Drogba for a goal, and at the start of the second half he nearly scored with an angled volley from Cech’s kick-out that flew narrowly wide. Nonetheless, West Ham took credit for thwarting Chelsea so often.

Premier LeagueWest Ham UnitedChelseaKevin McCarraguardian.co.uk

• Arsenal concede two goals from set-pieces
• Criticism of free-kick and penalty awarded to West Ham

Arsène Wenger lamented a “major missed opportunity” as Arsenal lost a 2-0 lead to draw 2-2 at West Ham United today but the manager reserved most of his ire for the referee Chris Foy, whom he accused of making “extremely generous” decisions for the home team.

The Frenchman could not understand how Foy could have penalised Abou Diaby for a challenge on the West Ham substitute Zavon Hines, which produced the free-kick that led to West Ham’s first goal. And, after Foy had waved away strong penalty appeals for a tackle by the Arsenal defender William Gallas on Scott Parker, Wenger complained at the penalty that Foy did award.

Alex Song nibbled at Carlton Cole’s heels and saw the West Ham striker go to ground to win the kick. Alessandro Diamanti, another substitute, beat Vito Mannone to the bottom corner to send Upton Park into a frenzy.

“Frankly, look at how many times he [Cole] went down in the game,” said Wenger. “I think Cole played very well but do you expect him to go down then? No. We were in control but as long as we didn’t score the third goal, the game was not over.

“West Ham continued to fight and we conceded two goals on two set pieces, two generous set pieces. West Ham is happy tonight and I can understand that. The free-kick and the penalty were extremely generous. I don’t think either should have been given. West Ham fought until the very last and the crowd got behind the referee’s decisions.

”It’s a major missed opportunity and, in our job, you don’t have the luxury to miss opportunities. You need to get points when you deserve to get them and, sometimes, when you don’t deserve them. We didn’t get them here when we deserved them.”

Gianfranco Zola, the West Ham manager, felt that Foy ought to have pointed to the penalty spot earlier than he did. “It looked, from my position, that the first one [Gallas on Parker] was more of a penalty but I haven’t seen the replays yet so I can’t be precise.”

Zola could also take issue with Foy for his controversial decision to dismiss Parker for a second bookable offence in the 85th minute. Parker was penalised for a deliberate handball, after sliding in to tackle Cesc Fábregas. “I have something to say about that,” said Zola, “because Scottie got the ball with his head. He was pushed as well.”

Zola felt that, having entered the game with only one point from the previously available 15, this draw “felt like a victory”. “Last week [at Stoke City] I saw something had changed and today, I have seen something more and that is very encouraging,” he added.

Premier LeagueArsenalArsène WengerWest Ham UnitedDavid Hytnerguardian.co.uk