Chelsea suffered their first Premier League defeat since the appointment of Guus Hiddink as they lost 1-0 at Swansea City on Saturday.
Hiddink replaced Jose Mourinho on an interim basis in December and has successfully guided Chelsea into the top half following a disastrous start to their title defence.
However, in the first match since Antonio Conte was confirmed as Mourinho's permanent successor, it was the Welsh club who celebrated reaching the fabled 40-point mark at the Liberty Stadium.
Gylfi Sigurdsson's fourth goal in five league matches gave Swansea a deserved lead midway through the first half, while Alexandre Pato wasted two golden opportunities to restore parity.
Having teed up the opener, Jefferson Montero headed narrowly over from a Sigurdsson cross late on, but Swansea held on to end Hiddink's 15-match unbeaten run and record their first Premier League win over Chelsea.
Francesco Guidolin, who is among the names linked with the Italy post that will be vacated by Conte following Euro 2016, will also now hope he has done enough to remain at Swansea beyond the end of the season.
Asmir Begovic was preferred in to Thibaut Courtois in Chelsea's goal, while Montero, who gave Branislav Ivanovic a torrid time during August's reverse fixture, was awarded his first start since Guidolin's appointment as head coach.
Oscar sent Chelsea's first chance high and wide, before a dangerous ball into the Swansea box was cut out to deny Ruben Loftus-Cheek a tap-in.
Angel Rangel should have done better at the other end when he was sent clean through on Begovic and failed to find the target, but Swansea did go in front in the 25th minute.
With Montero showing glimpses of the talent that caught the eye at Stamford Bridge on the opening day of the season, the Ecuador international created an angle to loft in a cross from the left wing.
Matt Miazga's weak clearing header fell kindly for Sigurdsson, who volleyed home his 11th Premier League goal of the season.
Swansea were on top and Leroy Fer arguably made the wrong decision to pass back across the six-yard box when a corner found him unmarked at the far post, before a Miazga interception inadvertently forced Begovic into a save.
A mix-up between Begovic and Baba Rahman on the edge of the box went unpunished by the returning Andre Ayew, but Montero was making the most of his chance in the starting XI as he drew yellow cards for Cesar Azpilicueta and Miazga.
It should have been 1-1 at the interval, but Pato blasted wide from close range after Oscar had beaten the offside trap to get in behind the Swansea defence.
Begovic reacted angrily when he saved at the feet of Alberto Paloschi early in the second half, feeling the Italian went in too strongly following a heavy first touch, before Pato squandered another great chance to pull level.
The Brazilian attempted to apply a clever finish to Pedro's ball from the left but only succeeded in diverting wide of the right-hand post.
The half-time introduction of Kenedy, which forced John Obi Mikel into the back four, failed to prompt an upturn in fortunes for Hiddink's men, and Jack Cork's long-range strike was kept out by a diving Begovic.
Montero's 75th-minute header was as close as either team came to adding to the scoreline thereafter, leaving Swansea to collect all three points and edge closer to safety.