Captain John Eustace's winner after just 43 seconds kept Watford's slim play-off hopes flickering after a poor game against Barnsley at Vicarage Road.
Real urgency from the Hornets, who must win all their remaining games to stand a chance, was rewarded as they took the lead in less than a minute.
Troy Deeney hit the bar with a powerful header from Don Cowie's corner before Eustace prodded in from close range.
Barnsley's best chance came when Bobby Hassell fired over from a late corner.
The mid-table visitors turned in a performance typical of a side with nothing to play for against Malky Mackay's men, who are now five points behind the top six with nine points to play for.
Soon after the early goal, Luke Steele had to save another Eustace header as the hosts made a flying start before Matt Hill nodded wide at the other end.
Barnsley did well to contain top scorer Danny Graham and when he did find space to fire in a decent effort, defender Kieran Trippier made a brave block.
Graham then chipped in a cross for strike partner Marvin Sordell just before the break but the youngster headed over the top.
Reuben Noble-Lazarus finally tested Scott Loach in the Watford goal with a 25-yard drive before Jacob Butterfield's corner was slashed over the bar by Hassell as Barnsley mounted some late resistance.
Watford held on to claim the points but with games against Championship leaders QPR and strugglers Preston to come, their chances of gatecrashing the top six look slim.