Arsenal blew the race for a top-four Premier League finish wide open as first-half goals from Alexandre Lacazette and Laurent Koscielny saw the Gunners beat Chelsea 2-0 at the Emirates Stadium.
Chelsea remain in pole position for a return to Champions League football next season in fourth, but are now just three points ahead of Arsenal and a surging Manchester United, who beat Brighton earlier on Saturday to stretch caretaker manager Ole Gunnar Solskjaer's winning streak to seven games in all competitions.
Maurizio Sarri's men risk being caught after a fourth defeat in 11 league matches that underlined the need for a striker, as the club close in on signing Gonzalo Higuain on loan from Juventus.
Sarri again opted to start without a recognised centre-forward, with a largely anonymous Eden Hazard preferred in middle of a front three against an Arsenal defence that had kept just one Premier League clean sheet since September.
By contrast Arsenal boss Unai Emery's selection was vindicated, with Mesut Ozil on the bench for the whole match and a switch to a 4-4-2 that allowed Lacazette and Pierre-Emerick Aubameyang to form a potent partnership up front.
Aubameyang should have opened the scoring after just five minutes when perfectly picked out by Lacazette's low cross, but he dragged his shot wide.
Kepa Arrizabalaga then produced a brilliant save to deny Koscielny from point-blank range, but it counted for little as from the resulting 14th-minute corner Arsenal took a deserved lead.
Lacazette did well to stay on his feet as he skipped past Marcos Alonso and Pedro Rodriguez before smashing the ball past Kepa at his near post.
Aubameyang should have doubled Arsenal's lead moments later but fired too close to Kepa when clean through.
Chelsea's start was horribly reminiscent of the 3-1 defeat at Tottenham in November that began their dip in form after a fine start to Sarri's reign at Stamford Bridge.
However, Arsenal's defence remained vulnerable when put under pressure and Pedro could have levelled when his lob over Bernd Leno shaved the outside of the post.
Aubameyang came close again, this time in spectacular fashion with an overhead kick that flew inches wide with Kepa beaten.
Chelsea were repeatedly caught out by Arsenal's well-worked set-pieces and paid for it again, albeit in fortunate fashion for the Gunners, six minutes before the break.
Sead Kolasinac headed down Lucas Torreira's free-kick into the path of Sokratis Papastathopoulos and his cross picked out the unmarked Koscielny.
The Arsenal captain mistimed his header, but the ball bounced in off his shoulder for his first goal since he ruptured his Achilles in April.
Chelsea could have got a foothold in the game before half-time as David Luiz's header came back off the post.
But in keeping with so many of their performances of late, Chelsea created little thereafter despite dominating possession.
Olivier Giroud was given a warm welcome back to the Emirates when he entered as a substitute, but even with the French international as a focal point, the visitors were toothless.
Sarri will hope Higuain, who scored 38 goals when the pair first worked together at Napoli in the 2015/16 season, is the answer.
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