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 on Saturday.
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.
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.
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.