Meetings with Chelsea have provided Manchester City with a measuring stick over the past 12 months.
In June 2020, at Stamford Bridge, City's 2-1 defeat handed the Premier League title to Liverpool, ending Pep Guardiola's two-season stay at the summit.
When City then returned to the same stadium in January, facing Chelsea for the first time in 2020-21, they rediscovered their mojo.
It was the fourth match in a sequence of 21 straight wins in all competitions and arguably the pick of the bunch.
Slick City, missing a host of stars due to COVID-19, swept Chelsea aside in a 3-1 win as Ilkay Gundogan, Phil Foden and Kevin De Bruyne all scored. They left London in fifth but firmly back on track.
It has since looked as though that might be a defining display in a historic quadruple achievement. City, like Chelsea, reached the Champions League semi-finals this week. They are already clear at the top of the league table and have an EFL Cup final Wembley date with Tottenham next weekend.
But in the FA Cup, the fourth competition, City were tasked with again taking on Chelsea, an entirely different prospect now Thomas Tuchel has replaced Frank Lampard and fortified the Blues.
And Saturday's semi-final saw City finally come unstuck as Chelsea claimed a superb 1-0 success.
Blues a different beast
The improvement in Chelsea from January's match to this game was evident even in a first half in which they managed only two legal shots.
All three of City's goals had come in the opening 45 minutes last time, tearing through Lampard's men at will. They were now limited to three first-half efforts of their own worth a combined 0.1 expected goals. Parity suited Tuchel, who could not allow De Bruyne to dictate once more.
At the other end, a portender for City's later downfall appeared in the 'offsides' column, for the Premier League leaders were warned long before Hakim Ziyech's 55th-minute breakthrough.
With just six minutes on the clock, Timo Werner advanced up the left and the flag stayed down. The forward played a low, square ball, which City could not cut out, and Ziyech scored. Then the flag went up. A let off.
When Werner exploited the same space 10 minutes after the interval, set clear by a gorgeous Mason Mount pass, goalkeeper Zack Steffen decided to act.
Seemingly unimpressed by the way with which a usually sturdy City defence had allowed Werner to centre and Ziyech to finish on the first occasion, Steffen advanced and fared no better than his team-mates. The United States international failed to narrow the angle and simply granted Ziyech an open goal when Werner made his pass again.
KDB blow for treble bid
Mount moved uneasily as he was replaced 15 minutes later, but the damage was done. City had already lost De Bruyne - who completed only 10 passes in the Chelsea half - to an ankle complaint at the start of the second half. That setback could have implications far beyond this encounter.
Steffen twice saved City, blocking from Ziyech and reaching a tame Werner prod, and De Bruyne's replacement Foden sought to muster up more magic.
Momentum swung but the scoreline did not. Ruben Dias headed over from close range and Raheem Sterling blasted beyond the crossbar.
Although the flag was raised again to deny Chelsea a second in stoppage time - Christian Pulisic, on for Mount, frustrated - the Blues battled, blocked and bellowed their way across the finish line.
Tuchel five times faced Guardiola in Germany and failed to end on the winning side. Boosted by spirit in defence and speed in the form of the much-maligned Werner, he finally found the formula.
The Chelsea coach will get another go at Guardiola in the league on May 8, a third meeting this season hot on the heels of the sides' respective Champions League semis. They will know by then if there is to be a further part to this epic in a European final - hopefully, for City's sake, with De Bruyne back involved in Istanbul.
Should Guardiola's men win that prize, the most precious of all, it could clinch a tremendous treble, but FA Cup glory is not on the agenda this season.
"We never speak about the four titles," the City manager said in midweek. "One game at a time."
The next game ensured nobody outside the club could speak about that clean sweep either - at least for another year.
