Rickie Lambert celebrated his new Southampton contract in some style by smashing home an exquisite free-kick to secure all three points, after Jay Rodriguez's opener had been cancelled out by a John Terry header.
The 31-year-old has established himself as a firm fans' favourite at St Mary's since joining from Bristol Rovers in August 2009 and has seen his stock rise considerably during his debut top-flight season.
Lambert was rewarded for his performances with a new deal until 2016 during the international break and repaid Southampton in spectacular fashion on Saturday afternoon.
Making his 500th career league appearance, the Liverpudlian curled home a spectacular 25-yard free-kick to rack up his 14th league goal of the campaign - more than any other English player in the Premier League.
Saints had gone ahead through another in-form striker Jay Rodriguez, who collected a neat one-two from Steven Davis before slotting home in the 23rd minute.
John Terry was allowed to level unmarked from a corner 10 minutes later, only for Saints to hit back immediately through Lambert's fine strike.
Chelsea peppered the Southampton goal throughout the second half but were unable to find a way past substitute goalkeeper Kelvin Davis, leading to a defeat that drops Rafael Benitez's side to fourth.
The west Londoners made no fewer than seven changes for Saturday's game and struggled to get a foothold on proceedings early on.
Rodriguez got Southampton's first shot away after less than two minutes and was proving a thorn in the visitors' side early on.
The £6million summer signing has looked rejuvenated since Mauricio Pochettino arrived and his new-found confidence was epitomised by a fantastic dink over Terry's head on the halfway line, before bursting through on goal and testing Petr Cech.
Rodriguez should instead have perhaps played the ball out wide to Jason Puncheon, who soon threatened himself, only to be denied by the onrushing Cech.
Chelsea were struggling to cope with Southampton's attacking intent and Nathaniel Clyne, linked this week with a move to Stamford Bridge, saw a cross-shot blocked by Branislav Ivanovic.
Lambert saw his follow up thwarted but it did not take long for the hosts to get the opener they deserved.
Rodriguez collected the ball after neat team play and play a fine one-two with Steven Davis, before slotting past Cech.
But while Chelsea were far from their best, they were also proving a threat on the counter.
That tactic almost paid dividends when a Maya Yoshida mishap put Fernando Torres through on goal. The Spaniard showed good poise and skill to delicately chip the ball past Artur Boruc, only for his goal to be chalked off for a handball in the build-up.
The disallowed goal kicked off a period of Chelsea dominance and, after Victor Moses saw a cross-shot deflect behind off Luke Shaw, they snatched a 33rd-minute leveler.
Marko Marin sent in the resulting corner and kamikaze defending allowed captain Terry to head home unmarked from six yards.
Chelsea were level for less than two minutes, though, thanks to a moment of magic from Lambert.
After Steven Davis was fouled by Ryan Bertrand, the 31-year-old stepped up to fire home a thunderous 25-yard free-kick into Cech's top-right hand corner.
Saints continued to press as the half came to a close and returned for the second period in similarly-impressive fashion, with Clyne testing Cech at his near post moments after the restart.
Chelsea, though, were an improved outfit going forward and Kelvin Davis, on at the break for the injured Boruc, saved a Frank Lampard strike well at his near post.
Clyne blocked a Marin effort as the west Londoners continued to knock on the door, with Kelvin Davis doing fantastically to tip a deflected Moses cross onto the bar and over.
Eden Hazard was introduced as Chelsea searched for another goal as play swung from end to end, with Rodriguez heading over before Jack Cork stopped Lampard.
Moses saw claims for a penalty against Yoshida waved away either side of efforts from Rodriguez and Shaw, before substitute James Ward-Prowse saw a squared ball to Steven Davis cleared at the last.
Anti-Benitez chants echoed around the ground as the 3,181 away supporters' frustrations heightened, with Hazard unfortunate to see a strike fly across the face of the goal.
Lampard tried to score with a backheel and then a free-kick as the Blues continued to press.
But, try as they might, there was to be no equaliser as Saints held on to secure an invaluable three points, much to Chelsea's frustration.