Premier League champions Leicester were plunged deeper into relegation trouble as they were beaten by Swansea, whose vital victory gave their own hopes of survival an enormous lift.
After a cagey start, Alfie Mawson's thumping volley and an incisive team goal finished by Martin Olsson gave the hosts a commanding 2-0 half-time lead.
Leicester offered more resistance in the second half - substitute Islam Slimani was denied by a fine save by Lukasz Fabianski - but fell to a fifth successive defeat, increasing the pressure on manager Claudio Ranieri.
The Foxes, who are just one point above the relegation zone, are the only side in the top four English divisions without a league goal in 2017.
They are also the first reigning champions to lose five consecutive top flight matches since Chelsea in March 1956 and now find themselves embroiled in a congested relegation battle in which the bottom six teams are separated by just five points.
Winless in the Premier League in 2017 and without a goal in their previous five league outings, Leicester entered this fixture in apparent freefall.
Goalkeeper Kasper Schmeichel described their faltering title defence as "embarrassing" after last Sunday's 3-0 home defeat by Manchester United, while Wednesday's FA Cup replay win over Derby was preceded by a dreaded vote of confidence from the club's board for manager Ranieri.
The Italian cut a forlorn figure on the touchline at the Liberty Stadium, standing motionless as he watched his side surrender two goals in a potentially defining eight-minute spell at the end of the first half.
There was little Schmeichel could do to stop Mawson's brilliant swerving volley, but the goalkeeper was at fault for Swansea's second.
Attempting to launch a counter-attack, the Dane's throw landed at the feet of Swans midfielder Tom Carroll, who started a slick one-touch passing move which involved Fernando Llorente and Gylfi Sigurdsson and ended with Olsson, whose firm strike Schmeichel should have saved.
As impressive as the goal was from a Swansea perspective, it was indicative of Leicester's porous defence - a far cry from the solid backline which formed the foundation for their improbable title success last season.
Swansea's resurgence under Clement
Despite starting the day a place below their opponents, Swansea's resurgence under new head coach Paul Clement was in striking contrast to Leicester's decline.
The Swans had won three of their five league games since Clement's appointment on 2 January, lifting them off the foot of the table and out of the bottom three to earn the former Derby boss the Premier League manager of the month award.
That accolade is meant to carry something of a curse - with managers often losing their next game after receiving the award - but Clement avoided such a jinx as he oversaw a polished performance.
Swansea are far more organised defensively than they were under predecessor Bob Bradley, with the defence and midfield now structured and disciplined with and without the ball.
The home side's energetic pressing gave Leicester no time to settle, and their two brilliant goals gave them a firm foothold in the game they never looked like losing.
A fourth win from six league games under Clement means Swansea climb up to 15th place, four points clear of the bottom three and with renewed hope of avoiding relegation.
Man of the Match - Alfie Mawson (Swansea City)
'Unbelievable' - the managers
Swansea City boss Paul Clement: "We have had a really good start and I'm very pleased with the players. We totally deserved that victory.
"The goal before half-time put us in strong position, we were solid all of the game. We had a couple of moments around 60/61 minutes where Leicester threatened but otherwise we were good."
Leicester City boss Claudio Ranieri:"Unbelievable. We started well. We wanted to make a good result against another team near the relegation zone. We make something good but the first shot on goal they score and then the second again. From there it was very difficult to get back.
"Our mind is on the Premier League. The FA Cup and Champions League is something different. We want to play well and be safe in the Premier League. Our main target is to be safe in the Premier League."
When will Leicester score again? - The stats
Leicester are the first reigning top-flight champions to fail to score in six consecutive league matches.
The Foxes have gone over 10 hours without scoring in the Premier League, 610 minutes.
No team in the top four tiers has won fewer points in 2017 than Leicester (one, level with Aston Villa, Coventry and Leyton Orient).
Gylfi Sigurdsson has been involved in eight goals in his last eight home Premier League games (three goals, five assists).
No defender has scored more Premier League goals in 2017 than Alfie Mawson (three, level with Marcos Alonso).
Leicester have kept just two clean sheets in their last 18 Premier League games.
By the time Leicester City start their next Premier League game, they could be bottom of the table. The Foxes host Liverpool on Monday, 27 February (20:00 GMT) - with all three teams below them in action before then.
Swansea's next game is a trip to leaders Chelsea in the Premier League on Saturday, 25 February (15:00 GMT).