As long as a season is and as much as experienced managers like Arsene Wenger and Rafa Benitez rightfully point out that it's a marathon, it is remarkable how swiftly an entire mood can change.
Just a few weeks ago, after all, it would have been difficult to predict anything other than a Chelsea win at home against Arsenal. Because, for all the angst surrounding Rafa Benitez's appointment, they've still possessed an admirable capacity to occasionally click and really turn it on - as perfectly illustrated against Aston Villa and away at Everton.
More relevantly, those performances indicated an extra level that Arsenal haven't really been able to reach. By contrast, in fact, Wenger's side have seemed to stagger through this campaign since their first big slump in October, only very rarely hitting anything like convincing form.
One of the moments in which they did, however, arrived on Wednesday. The second half against Swansea was arguably Arsenal's finest display of the season so far, with only Michel Vorm's brilliant goalkeeping and Theo Walcott's poor finishing preventing an utter siege from being converted to a severe beating.
And, although it threatened to look like the usual Arsenal story of passing and persevering, but never pushing over the line, Jack Wilshere ultimately produced the type of dynamic-shifting late goal that can change seasons. It certainly changed the mood.
At almost exactly the same time Southampton were pulling back a 2-0 deficit against Chelsea. The comeback and eventual 2-2 draw turned what had been an increasing issue for Benitez at Stamford Bridge into an outright problem. Chelsea have now only won two of seven games at home since the Spaniard took over, and those amid an awful lot of crowd anxiety.
In fact, although Arsenal haven't exactly enjoyed too many of their trips to the Bridge over the past decade, there is an argument they might be more comfortable there on Sunday than the home side.
As such, this feels like the kind of game that is genuinely on a level playing field, and one that may be unusually open.
For Arsenal, of course, it presents an opportunity to open up the Champions League chase too. A win would put them within five points of Chelsea with a game in hand. For Benitez's side, meanwhile, victory could go a long way to shutting Arsenal out of that race.
To do that, though, they may have to shut all the negativity around Stamford Bridge out. So far, that hasn't proven so easy. This is a game with a lot of emotion swirling around and even more implications.
Chelsea player to watch: Demba Ba A few weeks ago, Wenger revealed that he had considered a bid for Ba, only to eventually opt for Olivier Giroud. "It was one or the other one," the Arsenal manager said. The Chelsea striker now has the chance to again illustrate that picking Giroud may have been the wrong option. Although clearly a good player who often suggests a brighter future at Arsenal, the French forward has yet to offer a truly complete performance for Arsenal when necessary.
By contrast, Ba almost immediately put Chelsea's previous striker problems into context. Even if he does have to work a bit of a tandem act with Fernando Torres on Sunday, he may well do the same for Wenger. Not for the first time, Arsenal's defence will find it difficult to pin him down.
Arsenal player to watch: Jack Wilshere Quite simply, it'll be difficult to look at anyone else in the Arsenal team, most obviously because he commands attention with the manner he dictates play. While the previously excellent Santi Cazorla has started to suffer from fatigue of late, Wilshere finally appears to have fully recovered from that long injury and is approaching his very best. Wenger certainly thought so after the Swansea win on Wednesday.
Moreover, though, Wilshere also seems to bring out the best in Arsenal. When placed in a proactive midfield and given sufficient protection behind, it is as if he pins the entire team together. Even when they were at their most shapeless, as against Manchester City, Wilshere still illustrated a defiance that gave them a fighting chance. If Arsenal are to get anything out of Sunday, then he will likely be central to it - in every sense.
Key battle: Juan Mata v Arsenal's defensive midfield With Mikel Arteta injured and Abou Diaby still not at full fitness, any Arsenal gameplan could well be undone by their lack of cover in front of the defence - particularly with Mata in such exceptional form. As it is, Wilshere may have to drop back into that area to aid his team, thereby removing some of the vigour from Arsenal's attack too. This could well end up one of those individual tactical quirks that turns an otherwise even contest into an onslaught, with one side just exploiting the same flaw over and over again. It is imperative Arsenal shore up this area.
Trivia: The only player to have scored for two different clubs against Arsenal in a Premier League season was Darren Bent in 2010-11, hitting strikes for Sunderland and Aston Villa. Having scored twice against Wenger's side for Newcastle on December 29 before moving to Chelsea, Demba Ba can become the second.
Stats: Rafa Benitez has failed to win any of his last six Premier League matches against Arsenal, with his last win coming in Liverpool's 4-1 rout back in March 2007. Were Arsenal to claim three points, though, it would be the first time they will have won successive league games at Stamford Bridge - having won there 5-3 in October 2011 - for the first time since 1997.
Odds: Chelsea (1.8), Arsenal (4.33) and the draw (3.6) are on offer at Bet365, with Demba Ba first to score at 4.5.
Prediction: Although the anxiety around the two sides may bring the kind of uncertain game that can produce a decisive goal at any moment, it could also result in the kind of tense stalemate that sees both sides determined to just avoid defeat. As such, a draw seems likeliest.