Michael Tonge's deflected strike gave struggling Sheffield United's Premiership survival bid a crucial boost against relegated Watford.
Foster could do nothing to keep out Michael Tonge's strike
Tonge cut in from the left and drilled in a shot which deflected off Chris Powell's shoulder and past Ben Foster.
The Blades dominated the first half but were nervy throughout and came under siege in the final 15 minutes.
Watford's best chance fell to Marlon King who hit the bar with a looping lob from close in, but the hosts hung on.
The win moves Sheffield United up to 15th, three points clear of the relegation zone after defeats for Wigan and Charlton.
Blades boss Neil Warnock pre-warned supporters that he did not expect a classic encounter between the two Premiership strugglers, and his prediction was realised in an uninspiring clash at Bramall Lane.
United were anxious throughout and were constantly weighed down by shackles of relegation and the importance of taking three points.
Watford, with nothing to play for but pride since their relegation to the Championship last weekend, created chances to worry the home side but failed to find the quality to punish their opponents.
It took 15 minutes for the Blades to muster their first chance. Christian Nade latched on to Derek Geary's long ball and rounded Foster only for the England stopper to recover and smother the striker's effort.
Clarke Carlisle, who had already been carded, was lucky to not to be sent off when he elbowed Jon Stead while challenging for a high ball. Danny Shittu was controversially booked in the aftermath in a case of mistaken identity by referee Martin Atkinson.
With two minutes of a wretched first-half left, Sheffield United took the lead through Tonge's fortunate strike.
Powell hurled himself in the path of Tonge's effort only to deflect the drive over the diving Foster into his own net.
Blades found their edge in the second half with Chris Morgan heading narrowly over from Keith Gillespie's pin-point cross and Nade a constant threat up front.
Watford offered a spirited resistance as King muscled his way past two challenges and looped his effort over Paddy Kenny only to see his effort bounce off the bar.
In the last quarter, Watford pegged United back into their own half and pushed numbers forward in search of an equaliser.
But Henderson's long-range drive, Hameur Bouazza's slashed drive and Alhassan Bangura's last-gasp volley failed to break United's resolve.
Sheffield United boss Neil Warnock:
"We fought very hard and there were a lot of plusses.
"There was great determination from my players, they put their bodies on the line and make many blocks.
"They showed that we want to stay in this league. We had all our fans behind us and they were great - they have not turned us once this season."
Watford boss Adrian Boothroyd:
"We knew it was going to be a battle and we showed a great attitude considering that we were relegated last week.
"They were nervous and we should have taken advantage of that.
"But they did not have to work hard for their goal but we aim to finish the season strongly as we are starting to plan for next season."
Sheff Utd: Kenny, Geary, Morgan, Kilgallon, Armstrong, Gillespie (Davis 88), Jagielka, Montgomery, Tonge, Nade (Kazim-Richards 84), Stead.
Subs Not Used: Shelton, Webber, Alan Quinn.
Goals: Powell 44 og.
Watford: Foster, Doyley, Carlisle, Shittu, Powell (Chambers 75), Williamson (Hoskins 89), Bangura, Mahon, Bouazza, King, Priskin (Henderson 54).
Subs Not Used: Lee, Avinel.
Booked: Mahon, Carlisle, Shittu, Henderson.
Ref: M Atkinson (W Yorkshire).