Javier Hernandez punished Chelsea's wastefulness to secure a 1-1 draw for West Ham at Stamford Bridge on Sunday.
The Mexican whipped home a fine strike just four minutes after stepping off the bench to secure a precious point in the Hammers' fight for survival.
How David Moyes' side were even in a position to salvage something from the game will give Antonio Conte sleepless nights, given his side's total control until that point.
Eden Hazard and Willian were in mesmeric form from the outset and led the Hammers' defence a merry dance throughout, but Chelsea's dominance was not reflected in the scoreline, with Joe Hart enjoying a fine game in West Ham's goal.
Alvaro Morata saw goals ruled out in either half for offside, while Willian missed a host of glorious openings as the Blues failed to add to Cesar Azpilicueta's first-half opener and put the game beyond their opponents' reach.
The result means Chelsea are now a whopping 10 points behind Tottenham in fourth, while West Ham move six points clear of the relegation zone.
After a rousing and emotional minute's applause for former player and assistant manager Ray Wilkins, who passed away this week, the hosts started in confident mood.
Hazard was at his mesmerising best after just four minutes, weaving past a host of Hammers players before flashing a shot agonisingly wide of Hart's right-hand post.
Moyes' side weathered the early storm, however, and could have taken the lead themselves in the 14th minute when Marko Arnautovic pulled down a long ball in the penalty area. His goal-bound effort was well blocked by Gary Cahill, making his first start since 10 March.
Willian had a fierce drive diverted behind before Morata saw a goal ruled out after straying into an offside position to receive Hazard's incisive pass.
The same pair combined moments later, but Morata was unable to direct his back-flick from the Belgian's cross into Hart's far corner from close range.
Willian then twice came close to giving his side the lead. Firstly, he whipped a 25-yard effort past Hart's post and was then denied by the England goalkeeper's outstretched leg after latching onto Hazard's superb pass.
Chelsea's utter dominance was eventually rewarded nine minutes before the interval, when Morata's knock down found Azpilicueta inside the six-yard box and the Spaniard took a touch before prodding past a static Hart.
7 - Cesar Azpilicueta and Alvaro Morata have combined for seven goals in the Premier League this season; no other teammate duo have combined for more (joint-high with Salah-Firmino and Salah-Mane). Compañeros. pic.twitter.com/4GBbs4fuqx — OptaJoe (@OptaJoe) April 8, 2018
Chelsea's swagger initially continued after the interval, with Willian dragging a shot just wide and Morata heading over from a corner.
The Spanish striker saw his second effort of the game ruled out for offside on the hour after tapping in Willian's cross, while Victor Moses drilled a fierce strike past Hart's charmed goal.
They were made to pay for those misses when Hernandez expertly stroked home from Arnautovic's pull back to draw the Hammers level with 17 minutes remaining.
Conte's side threw everything forward in search of a winner, but found Hart in inspired form.
The on-loan keeper palmed Marcos Alonso's powerful drive over and then tipped substitute Olivier Giroud's header onto the post during a dramatic finale, in which the Hammers ultimately held firm.
Key Opta Facts:
- Chelsea failed to win a Premier League game at Stamford Bridge under Antonio Conte in which they led at half-time for the first time, having won all 18 of their previous games under the Italian when ahead at the interval.
- West Ham avoided defeat in an away Premier League game against the reigning champions for only the third time, having beaten Man Utd in 2001-02 and drawn 2-2 with Chelsea in 2015-16.
- Cesar Azpilicueta scored his fifth Premier League goal for Chelsea in his 191st appearance – all five of his goals have been against teams beginning with W (two against Watford, two against West Brom and one against West Ham).