Swansea ended Chelsea’s unbeaten Premier League run under Guus Hiddink with a 1-0 victory at the Liberty Stadium on Saturday.
Chelsea came into the game unbeaten in 15 games, but Gylfi Sigurdsson's goal on 25 minutes was enough for the hosts, who have moved onto 40 points in the Premier League.
Only Harry Kane (11) and Sergio Aguero (10) have now scored more Premier League goals in 2016 than Sigurdsson (9).
The visitors could have no complaints about the result although Pato, who was making his full debut, missed a glorious chance from six yards out in the second period.
Chelsea broke the news in the week that Antonio Conte is set to take over as manager in the summer and although he wasn't in attendance in south Wales, he is reportedly keen to give Asmir Begovic a chance to impress.
Begovic came in as one of three changes for Chelsea to replace Thibaut Courtois, his first start since November in the Premier League.
Swansea have shot up the league under the watch of their new manager Francesco Guidolin, conceding just two goals in five home games under the Italian, and the hosts started on the front foot.
Jefferson Montero, who terrorized Chelsea when these teams met in August, provided a great outlet on the left and it was from his tricky work that Sigurdsson opened the scoring on 25 minutes.
Montero jinked his way into the box before lofting in a cross that Matt Miazga could only head straight into the path of Sigurdsson, who hit a first-time effort into the bottom corner. It was his 11th goal of the season.
The goal pumped Swansea up to another level as Chelsea struggled to handle their intensity without the ball. Leroy Fer couldn't turn home a decent opportunity at the back post on 35 minutes before a mix-up between Asmir Begovic and Baba Rahman nearly presented a chance to Andre Ayew but he fluffed his lines.
For all their positive play, Swansea nearly went in at the break level as Pato should have done better from 10 yards after Oscar's shot was blocked.
The Brazilian could have been forgiven for missing that chance but the one he spurned on 53 minutes was harder to excuse.
Pedro did brilliantly down the left and put a cross seemingly on a plate for Pato to finish but his decision to try an audacious flick didn't come off as the ball spun wide of the post.
Jack Cork stung the palms of Begovic three minutes later before Montero somehow headed the ball over the crossbar from six yards after a brilliant dribble and cross down the flank by Sigurdsson.
Hiddink tried to freshen things up by bringing on Bertrand Traoré and the retuning Radamel Falcao, but they looked incapable of severely testing the Swansea back four, led impeccably by Ashley Williams, who made two brilliant clearances inside his area in stoppage time.
Matt Le Tissier's Soccer Saturday verdict
"Chelsea were outfought and out-battled today. Swansea just had much more desire than Chelsea had throughout the game. They were just quicker to second balls all over the pitch and thoroughly deserved the three points.
"Chelsea dominated possession a little bit more but for me Swansea were well worthy of the three points."
Swansea: Fabianski (7), Rangel (7), Fernandez (7), Williams (8), Taylor (7), Cork (7), Fer (7), Sigurdsson (8), Ayew (7), Montero (8), Paloschi (8)
Subs used: Naughton (6), Routledge (7), Gomis (5)
Chelsea: Begovic (7); Azpilicueta (6), Miazga (5), Ivanovic (6), Baba Rahman (6); Mikel (6), Fabregas (4); Pedro (6), Loftus-Cheek (7), Oscar (6); Pato (5)
Subs used: Kenedy (6), Traore (6), Falcao (6)
Man of the match: Ashley Williams