Manchester City's defeat at Stamford Bridge ended their reign as Premier League champions with a full seven fixtures left until the end of the season.
The visitors finished the game with 10 men after Fernandinho's sending off and Pep Guardiola will be left facing many questions over his side's underwhelming title defence.
The night before, Liverpool had delivered perhaps their best home performance of the campaign, pummelling Crystal Palace to move a staggering 23 points clear.
There is still some sorting out to do elsewhere, not least with European places and relegation still to be decided.
And in the chase for Champions League football – City's Court of Arbitration for Sport appeal notwithstanding – Manchester United did their cause no harm with a fine win over a Sheffield United side in danger of undoing the progress they made before the coronavirus-enforced shutdown.
Arsenal, meanwhile, stopped the rot with a win away at Southampton and had two young homegrown talents on the score sheet to lift their fans' spirits.
Below, we analyse the tactical trends within a landmark week in the history of the Premier League...
1) Fernandes & Pogba pull the strings so Rashford & Martial can shine
It is certainly no coincidence that Manchester United’s best possession-centric performance under Ole Gunnar Solskjaer to date coincided with Bruno Fernandes and Paul Pogba starting together in central midfield for the first time.
With a rejuvenated Nemanja Matic cutting passes into the two creative midfielders like a considerably younger version of himself, Man Utd showed inventiveness in the middle that created space for Marcus Rashford and Anthony Martial.
Getty Images
Their most common tactical shape saw Rashford come narrower than usual from the left and Martial drop off the front line, creating a tight square with Fernandes and Pogba in central attacking midfield.
From here, quick-tempo short passes overwhelmed the Sheffield United midfield, particularly with Luke Shaw and Aaron Wan-Bissaka providing the width to force the visitors into a flat 5-3-2.
In that formation, pockets of space inevitably open on the outside of the three-man midfield, and it is here that Rashford and Martial combined for the first and third goals. Man Utd recorded a 2.57 Expected Goals (xG), their highest against a top-10 club in almost three years.
2) Guardiola’s striker-less system fails as KDB takes up strange position
Chelsea deserve credit for their victory on Thursday night, but there was an all-too familiar vulnerability about Manchester City – physically weak, lacking quality at left-back, easy to counterattack – that they must fix with new signings this summer.
However, Pep Guardiola was also to blame for switching to the 4-6-0 formation that beat Real Madrid earlier this season; Bernardo Silva had no impact whatsoever as a false nine.
Getty Images
The idea behind a striker-less formation is to confuse the centre-backs who, with nobody to directly track, get caught by midfield runners in behind. But not only did City fail to make any of these runs, their entire team clustered into predictable midfield positions that allowed Chelsea’s low block to casually snuff out danger.
Silva drifted out to the left wing to get on the ball in harmless areas, while Raheem Sterling and Riyad Mahrez were too wide to affect the game. Meanwhile, playing Ilkay Gundogan and Rodri together in midfield significantly slowed their possession game down.
Most important of all, Kevin De Bruyne spent the first half in a bizarre left-centre role, 20 yards from play and sandwiched between Ross Barkley and Andreas Christensen. City improved dramatically after bringing on Gabriel Jesus and David Silva.
The lesson for Guardiola here is that a 4-6-0 won’t work against opponents who sit deep and absorb pressure.
3) Chelsea’s disciplined block and Pulisic-Abraham switch show Lampard is improving
As the 2019-20 Premier League season has progressed, Frank Lampard’s tactical acumen has steadily and visibly improved.
After struggling to compress space between the lines during his first few months in charge he appears to have settled on the methods of his old manager Jose Mourinho, once again going for a conservative low block against ‘Big Six’ opposition with targeted periods of counterattacks.
Getty
Having successfully limited City’s attackers in the first half and ruthlessly capitalised on a defensive mix-up to open the scoring, Chelsea really came alive in the second half after Lampard brought Tammy Abraham onto the pitch in response to Silva and Jesus stretching the game.
Lampard had noticed City were cranking up the pressure and, rather than double down on a defensive strategy, he increased Chelsea’s counterattacking threat. Pulisic and Abraham linked superbly on the break, while Chelsea’s midfield sought long balls over the top of a now-stretched City defence.
Lampard had sussed the game out, finding a way to gain a tactical advantage during the first-half stalemate and the second half’s more chaotic pattern.
4) Arteta successfully trials new 3-4-3 and 3-5-2 formations
It was an unspectacular performance and a quite forgettable win at St Mary’s, but for an unstable side like Arsenal that is worth celebrating.
Mikel Arteta’s men rarely looked in trouble, thanks in part to Rob Holding’s man of the match performance, but mainly because there was a steeliness and balance to the Gunners' new 3-4-3 formation.
Getty Images
Hector Bellerin and Bukayo Saka were asked to perform in box-to-box wing-back roles and did so with aplomb, regularly dropping to make a flat back five that completely stunted Nathan Redmond and Stuart Armstrong, in turn ensuring Danny Ings was a passenger in the middle.
The downside to the system was Nicolas Pepe being taken narrower than he would like, and it is worth noting that Arsenal only managed five shots on goal all match; a goalkeeping error is perhaps the only reason this game didn’t end scoreless.
However, Arsenal pressed more effectively than they have at any point since the restart and looked solid in their new shape, even improving in attack when Joe Willock replaced Pepe and Arteta shifted to a 3-5-2.
For games against ‘Big Six’ clubs, a three-man defence could be the way forward.
5) Liverpool provide timely reminder of their tactical variety
Chelsea’s win crowned Liverpool Premier League champions on Thursday night and – after a three-month break threatened to take the gloss off the moment – thankfully their most recent performance served as a timely reminder of what makes Jurgen Klopp’s side so special.
The relentless energy – both in the press and recycling possession from one flank to the other – was extraordinary to witness in the 4-0 destruction of Crystal Palace, who failed to get a single touch of the ball in the Liverpool penalty area.
The positional rotation in midfield and precision of their passing has formed the bedrock of their remarkable season. But equally important is the tactical variety in their attack.
Getty
On Wednesday, Palace had to deal with long balls straight over the top, full-backs playing dangerous crosses, the front three playing incisive one-touch football centrally, Sadio Mane and Mohamed Salah dribbling directly at the full-back, Virgil van Dijk dominating at set-pieces, Fabinho taking (and scoring) long shots, and Trent Alexander-Arnold firing in direct free kicks.
It is the variation in Liverpool’s attack that has made them impossible to predict and impossible to stop.