Go back a little over two months and Chelsea seemed in disarray. Their 3-0 defeat by Arsenal left them eighth, eight points behind leaders Manchester City.
Now Antonio Conte's team top the table. Victory at the Etihad Stadium today may nevertheless move them into a different position: That of the favourites to win the league, a mantle that still rests with Pep Guardiola's team.
Yet while City have drawn their last three home league games, they are inconsistent, while Chelsea's ascent has been based on remarkable, relentless reliability. They have won seven successive matches, keeping clean sheets in six.
It helps explain why this appears a clash of opposites. Chelsea are paragons of defensive excellence while City take chances and occasionally make mistakes at the back. Guardiola rotates his team whereas Conte could name the same side for the seventh successive game.
The Italian made a decisive tactical change, from 4-2-3-1 to 3-4-2-1 during the Arsenal game. The Spaniard can switch systems, sometimes within a match, rather than deciding on one and sticking to it.
Yet he has had to alternate. City had not had a free midweek all season until this week and, with a Champions League dead rubber against Celtic next, he has no need to rest anyone.
It means Guardiola has had time to devise and implement a tactical plan. That adds intrigue.
Given the importance he attaches to his wingers, it seems probable he will try and use them high up the field, to press and peg back Chelsea's wing-backs, as Tottenham did in the first half last week, or instead to exploit the space behind them.
It promises to be the most severe test of Victor Moses' defensive credentials.
The Nigerian was Chelsea's match-winner against Spurs. He forms just one of the subplots. Yaya Toure could cap his renaissance and return from exile with a start for City.
Pedro Rodriguez, promoted from Barcelona's B team by Guardiola in 2009, has been revitalised by Conte.
His combination with Eden Hazard poses questions of opponents who deploy a solitary deep-lying midfielder. Guardiola could play two, maybe with Ilkay Gundogan alongside Fernandinho, perhaps reviving a 3-2-4-1 shape.
Whichever, Guardiola's focus on attack promises the stiffest examination of a Chelsea defence that has conceded just once in 680 minutes, just as this could be a defining game.
Chelsea have beaten the teams in fifth and sixth, Tottenham and Manchester United, of late, but lost to two of the top four, Liverpool and Arsenal, before changing formation. City's biggest domestic scalp was United.
Now they enter a decisive December when they face each of their title rivals.
Victory would act as a resounding endorsement of either. It is time to judge both.
MANCHESTER CITY V CHELSEA
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