Southampton - Arsenal missed a chance to move four points clear at the top of the Premier League table following a 2-2 draw at Southampton on Tuesday.
Arsene Wenger's side were on course to cement first place in the English top-flight after recovering from the shock of conceding Jose Fonte's first half goal.
Olivier Giroud and Santi Cazorla had put Arsenal 2-1 ahead at St Mary's, only for Adam Lallana to equalise after the break and deny Arsenal a vital three points.
The Gunners are two points clear of second placed Manchester City, who can claim top spot with a victory at Tottenham on Wednesday.
Arsenal's frustration was summed up when French midfielder Mathieu Flamini was dismissed for an 80th minute challenge on Morgan Schneiderlin.
Ivan Gazidis, the Arsenal chief executive, had confirmed before this game that Wenger would soon sign a new, three-year contract to remain at the club.
An away victory would have been the ideal way to mark the announcement, but it quickly became clear this would be a major test for the leaders.
Southampton's defeat at the Emirates Stadium had triggered an alarming run that had brought just two wins from their previous 11 league games.
Injuries have taken their toll and with Rickie Lambert out and Dani Osvaldo suspended following a training ground fight with Fonte, Saints boss Mauricio Pochettino handed a first league start to teenager Sam Gallagher.
But that did little to unsettle the home side who started the game positively and continued to dominate throughout the first half, with Gallagher coming close to breaking the deadlock with a powerful shot after just seven minutes.
Fonte's 21st minute goal came when Luke Shaw, the Saints left-back who has been linked with a move to Manchester United and Chelsea, delivered a deep cross towards the centre-back, who headed in at the far post.
Lallana came close to adding a second when he forced Arsenal goalkeeper Wojciech Szczesny into a full length save and Gallagher stabbed an even better chance wide after Monreal's attempted clearance had been charged down.
The opening 45 minutes had been one-sided but Arsenal finally showed signs of their threat immediately before the break when Laurent Koscielny forced a superb reaction save from Artur Boruc.
But if that was a warning, Southampton failed to heed the signs and the visitors exploded into life after the restart with two goals in seven minutes.
Giroud brought the Gunners back on level terms in the 47th minute with a clever flick after Bacary Sagna had forced his way into the Southampton box before squaring for the France striker to redirect the ball into the net.
Then, with the home side reeling, Spanish playmaker Cazorla added the 52nd minute second with a low, left foot drive from the edge of the area after Mesut Ozil had led a breakaway.
Pochettino's side appeared deflated but they conjured the perfect response just 60 seconds later when Jay Rodriguez worked his way towards the byline before pulling back for Lallana who struck from 10 yards out.
Both sides appeared capable of going on to win the game but it was Arsenal who had the better chances to reclaim the lead.
Ozil capped a surging run with a powerful shot that was deflected over Boruc and rebounded to safety off the crossbar.
And Monreal should have done much better when he found himself with a clear shooting opportunity on the left hand side of the Saints box, only to blast his effort high into the crowd.
But the advantage swung back in Southampton's favour following Flamini's reckless, two-footed challenge on Morgan Schneiderlin in the 80th minute.
Referee Lee Mason had no hesitation in showing the Frenchman a straight red card and while Flamini protested his innocence, television replays supported the match official's decision.