West Ham ended Swansea's four-match winning streak with a 1-0 away success that reduced the gap at the top of the Sky Bet Championship to a single point.
Divine Mukasa's calm first-half finish proved the difference on an afternoon when a new attendance record of 20,973 was set at the Swansea.com Stadium.
After West Ham's 6-0 thumping of Lincoln in midweek, this was a far more tactical affair but the importance of the win will not be lost on boss Nuno Espirito Santo.
The Hammers climb to second, above Middlesbrough on goal difference, and to within a point of Swansea, who lost at home for the first time since April.
Swansea had made their best start since 1923 by winning seven and drawing two of their opening 10 games.
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The Welsh outfit also boasted the tightest defence in the Championship, while West Ham's 29 league goals at a rate of almost three per game was the highest in England's top four divisions.
There was a cup-tie feel to the occasion as the two teams battled for midfield supremacy, but openings were difficult to find.
The first shot of any description took 34 minutes to arrive - but the Hammers made it count.
Marko Stamenic was caught in possession and Pablo released Mohamadou Kante as Swansea defenders waited in vain for an offside flag.
Kante showed good composure on the by-line to pick out the unmarked Mukasa, who slotted home his third goal of the season.
It was only the third goal Swansea had conceded at home and the setback sparked the hosts in to action.
Adam Idah and Ronaldo had shots deflected wide in the space of a minute before Ben Cabango shook the underside of the crossbar.
Jay Fulton flighted a free-kick to the far post that was headed into Cabango's path by Moussa Yeo.
Cabango swivelled in a congested penalty area to strike the frame of the Hammers' goal, and Swansea were unable to force the ball home in the ensuing goalmouth scramble.
England international Jarrod Bowen had a quiet first half, but the Hammers skipper burst into life four minutes after the break to force a full-length stop from Lawrence Vigouroux.
It was the Hammers who were starting to look like league leaders as Vigouroux produced two excellent saves to deny Pablo and Bowen.
Idah fired just wide as Swansea rallied and Cabango made a hash of a bouncing ball in the penalty area as West Ham failed to clear Yeo's centre.
Substitute Stephen Eustaquio was off target and Idah headed over as Swansea began to ask serious questions of the Hammers' back line.
Bowen's mazy run deserved more than the careless finish from Manor Solomon that followed, but Ronald wasted a great chance to equalise in stoppage time.
The managers
Swansea's Vitor Matos:
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West Ham's Nuno:
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