Tommy Smith scored one and made another as Coca-Cola Championship strugglers Watford ended Swansea's long unbeaten run.
Roberto Martinez's side had gone 16 games in all competitions without defeat - a run stretching back to November - but, despite playing some trademark attractive football, they fell behind to two well-taken goals.
Smith first picked out Tamas Priskin to score at the far post and then ended a move he started with a powerful drive as the Hornets climbed out of the relegation zone.
Swansea dominated their FA Cup draw with Barclays Premier League side Fulham last weekend and started as they left off, controlling possession with a flurry of neat midfield passes.
But it was the hosts who engineered the first shot in anger, Jobi McAnuff and Smith combining nicely for Don Cowie to lash over the top from just outside the area.
Thomas Butler then called Hornets goalkeeper Scott Loach - superb against Chelsea in the FA Cup defeat on Saturday - into action with a curling effort from the left-hand side of the area before Jack Cork shot over at the other end following a bright break.
The Swansea midfield trio of Leon Britton, Joe Allen and Jordi Gomez were leaving Watford chasing shadows at times as they effortlessly moved the ball about but it was Brendan Rodgers' side who took a 23rd-minute lead.
Butler's corner was cleared to McAnuff and the pacey winger sped forward before slipping in Smith on the left of the area.
Smith sent in a low cross that was aimed at Cowie but the ball ran on for Priskin at the back post and the Hungarian kept his cool to slot home between Dorus De Vries' legs.
Priskin had come off the bench to put Watford ahead against Chelsea at the weekend and his calm finish made it four goals in four games.
The goal may have come against the run of play but it was two 10 minutes later.
Smith was again integral as he played in McAnuff down the left. His low cross found Cowie, who looked certain to score but his low shot from eight yards was brilliantly saved by De Vries.
Swansea defender Ashley Williams hooked the ball clear but only to Smith, who confidently drilled a half-volley into the bottom corner from 20 yards for his 13th of the season.
The Swans carved out a chance in the opening seconds of the second half but Nathan Dyer's low cross evaded everyone in the six-yard box.
Jason Scotland - whose strike against Fulham made it 12 goals in 12 games - whistled a free-kick narrowly over as Martinez's men pushed for an early goal that would give them a way back into the match.
The hosts were under the cosh but still managed to fashion a chance to seal the win in the 67th minute.
Cork played Priskin in over the top and, after he held off a challenge, the striker managed to square the ball for the onrushing Smith but under pressure he was unable to steer the ball home.
Martinez added an extra striker in the form of qualified teacher Guillem Bauza but the hosts held on to claim a vital three points that breathes life into their survival battle.
Teams
Watford Loach, Mariappa, DeMerit, Mike Williamson, Doyley,Cork, Cowie, McAnuff, Jenkins (Lee Williamson 53), Priskin,Smith.
Subs Not Used: Lee, Hoyte, Harley, Hoskins.
Booked: Cowie, Lee Williamson.
Goals: Priskin 23, Smith 33.
Swansea De Vries, Serran, Williams, Monk, Tate (Bessone 57),Britton, Gomez (Bauza 57), Allen, Butler, Dyer (Orlandi 82),Scotland.
Subs Not Used: Gower, Rangel.
Booked: Britton, Serran.
Att: 13,727
Ref: Anthony Taylor (Greater Manchester).
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