After beating Manchester City twice, across two competitions, and knocking Real Madrid out of the Champions League at the semi-final stage, a relentless Chelsea looked certain to secure a top-four finish in the Premier League.
Step forward Arsenal, having failed to qualify for the Champions League themselves, who ensured that their London rivals wouldn't have it all their own way in the final few weeks.
Emile Smith Rowe's first-half strike was the only goal in a 1-0 win at Stamford Bridge. The assist will go down as Pierre-Emerick Aubameyang's but it was Jorginho's calamitous back pass that allowed unfancied Arsenal the chance to open the scoring.
Chelsea's pre-Thomas Tuchel problems surfaced once again; their eight errors leading to goals is the worst total in the league aside from Liverpool, who have had nine.
There was little surprise that Chelsea's expected goals dwarfed that of Arsenal's (1.85 to 0.59) but they could not get the job done.
Kai Havertz missed a first-half one-on-one chance after a Gabriel error, while Kurt Zouma and Olivier Giroud both hit the bar within seconds of each other at the death.
As much as Mikel Arteta's side deserve praise for their fighting spirit, they were fortunate to face a flat Chelsea who, in truth, beat themselves. They didn't threaten enough, didn't look sharp, weren’t sufficiently quick in possession and appeared lacking in energy.
Equally, Tuchel admitted fault with his team selection, having chosen to rest key players like Timo Werner, Antonio Rudiger and N'Golo Kante ahead of the FA Cup final this weekend.
However, it came to light in Tuchel's post-match interview on Chelsea FC's Fifth Stand app that Kante was carrying a minor injury.
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"You can say now that this is a totally lucky win and nobody can argue with that, because it's not a deserved win for Arsenal," Tuchel said after the match.
"But I don't want to go there because we all lacked the right energy and focus, so we didn't deserve it. And maybe I gave some signals to the team with changes that Saturday is on my mind.
"Even if it's one per cent or five per cent, this is not how we approach things because you get punished. It's hard to swallow, hard to accept, but there's no other way."
Although Tuchel has been masterful with his rotation this season, he can't always get it right. His team selection perhaps gave a feeling that the FA Cup showdown against Leicester was more important than the game against the Gunners, but ultimately the team will be judged on their league position by the board – especially if they miss the top four.
Tuchel's mission when he was appointed in January was to finish in the Champions League places. That is in doubt again, despite a remarkable climb up the table in recent weeks.
Chelsea have only lost twice in the league under their new manager and beaten the likes of City, Liverpool and Tottenham. They have risen from 10th to fourth but failing now would no doubt still see the German coach looked upon unfavourably.
As well as the upcoming finals, the league games against Leicester and Aston Villa will be matches with a lot riding on them.
Meanwhile, Leicester will have a game against Tottenham on the final day of the season. Below them are Liverpool and West Ham. They could conceivably close the gap due to their more favourable run of fixtures to close out their seasons.
There's no need to go overboard, Chelsea remain the favourites to finish the job, but their vulnerability has been shown once again with a Lampard-era throwback performance.
It's rarely plain sailing for this club and they now have four cup finals, not two, to get themselves ready for.
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