Manchester City's Premier League title celebrations were delayed by a 2-1 defeat to Chelsea on Saturday in a rehearsal of this month's Champions League final.
Pep Guardiola's side are on the brink of a third championship in four seasons but could not complete the job at the Etihad Stadium after Sergio Aguero allowed Edouard Mendy to save a tame spot-kick.
That effort would have added a swift second to Raheem Sterling's breakthrough strike on the stroke of half-time.
Instead, Chelsea rallied after the interval and scored through Hakim Ziyech – their FA Cup semi-final hero against City – and then, in the final seconds, Marcos Alonso to boost their top-four aspirations and keep the leaders waiting, unable yet to turn their focus to the European showpiece.
Although Guardiola's puzzling 3-3-4 formation initially had Chelsea on the back foot, it was a defensive mishap rather than a tactical manoeuvre that forged the opening on 44 minutes.
Andreas Christensen misjudged a bouncing ball and was brushed aside by Gabriel Jesus, who squared for Aguero in front of an open goal, only for Sterling to sweep into the net when his team-mate dallied.
Aguero did not have to wait long for his next chance, as Christensen hobbled off and Jesus tangled with Billy Gilmour before Chelsea were able to introduce a substitute, prompting a City penalty.
However, as Thomas Tuchel fumed at the officials, a dreadful Panenka attempt from Aguero was gathered by Mendy.
The shortcomings of the City system were then laid bare shortly after the hour mark when Rodri – their sole central midfielder – lost out to Ziyech and left space for a Chelsea attack which ended with the same man arrowing a low finish beyond Ederson.
Chelsea pushed for a winner but were twice denied by the offside flag. Timo Werner had already strayed before netting in the first half and did so again, before a closer call kept substitute Callum Hudson-Odoi off the scoresheet.
City furiously appealed for a second penalty when Sterling went down under pressure from Kurt Zouma, but the final twist was in Chelsea's favour as Alonso's scuffed shot looped over Ederson.
What does it mean? Pep innovation uninspiring
Was this a tactical tweak designed to get City over the line in the Premier League, or a test of Tuchel ahead of the Champions League final? We are unlikely to find out until that fixture, but it did not work on this occasion.
Guardiola went from a line-up consisting almost exclusively of midfielders to a system with only Rodri holding the fort. He lost possession just five times but crucially had no support on those rare occasions, leading to the equaliser.
Tuchel gets to grips with Guardiola
In Tuchel's time as coach at Borussia Dortmund, Guardiola turned to a similarly outlandish formation in a crunch clash between BVB and Bayern Munich. The Catalan coach oversaw a 5-1 win.
This time, Tuchel adapted. An improved second half saw Chelsea exploit the space behind a non-existent City midfield, with Nathan Ake badly exposed time and again. Reece James created two chances from Ake's flank and Cesar Azpilicueta and Werner each supplied assists.
Tuchel has now got the better of Guardiola twice in three weeks and can head to the Champions League final full of confidence.
Forgettable farewell to this fixture
Aguero has typically enjoyed playing Chelsea; no player has more Premier League goals against the Blues than his 10, including two hat-tricks. But this was not a match he will reflect on fondly.
Sterling did not look convinced when the striker attempted to explain away his loose touch before the opener, while a cute spot-kick attempt only increased the City great's embarrassment. That was one of only two shots.
City will now be hoping Aston Villa can beat Manchester United on Sunday to clinch the title. Guardiola's men play again at Newcastle United on Friday. Chelsea, up to third, host Arsenal on Wednesday before next weekend's FA Cup final.