Rafael van der Vaart's long-range strike ensured a happy end to the week for Tottenham manager Harry Redknapp.
Van der Vaart beat Scott Loach with a dipping 25-yard shot, but Spurs had to survive a second-half onslaught to reach the FA Cup fifth round.
Spurs' preparations were disrupted by Redknapp's ongoing tax evasion trial .
Although Van der Vaart also hit the crossbar, Watford struck the woodwork twice in a dominant second-half showing and were unlucky not to force a replay.
Spurs, who last won the FA Cup in 1991, rested Gareth Bale, Ledley King and Aaron Lennon but still named a strong side which produced arguably their worst display of the season.
Did you know?
Before this win, Tottenham last beat Watford in the FA Cup in 1987. They went on to lose the final to Coventry
Watford responded marvellously to recover from Van der Vaart's opener against the run of play, and teenager Sean Murray and captain John Eustace struck the same post after the break.
The hosts worked hard from the off to deny Luka Modric and Rafael van der Vaart space and time and, for all Spurs' possession, their first shot on goal did not arrive until the half-hour mark.
Watford, on the other hand, twice threatened in the opening exchanges as midfielder Murray saw his low drive parried by Carlo Cudicini before Prince Buaben hit a swerving shot narrowly wide from 25 yards.
The home side's bold line-up saw forward Troy Deeney causing Danny Rose no end of problems on the Tottenham left, and his cross led to Joe Garner miscuing a clear chance in front of Cudicini.
Spurs were still struggling to make headway in the final third, with reported Newcastle target Adrian Mariappa impressive in the Watford defence, although Jermain Defoe did at least register an effort on target when he linked with Emmanuel Adebayor to shoot tamely at Loach.
Did you know?
Watford boss Sean Dyche scored a penalty for Chesterfield in their famous FA Cup semi-final defeat to Middlesbrough in 1997
Van der Vaart opened the scoring just before the break. Kyle Walker carried the ball from deep inside his own half to find the Dutchman in a rare island of space, and he lashed in a left-footed shot which bounced in front of Loach and into the net.
Redknapp introduced Lennon for the below-par Modric at the break but again it was the Championship side who went close to scoring.
Jake Livermore lost possession on the edge of his own penalty area following a sloppy pass from Cudicini, and when Murray was played in his shot was tipped on to the post before Deeney hit the rebound wide.
Spurs then had a dominant 10-minute spell during which Walker's header was turned over and Van der Vaart curled an effort against the crossbar after a breakaway led by Defoe.
Watford controlled the remainder of the game, with Eustace heading a corner against the post, Joe Garner nodding wide and Nyron Nosworthy seeing his low shot kicked off the line by Cudicini.
But Tottenham clung on to record the win which puts them into the hat for the last 16 and leaves Watford to concentrate on Championship survival.
Live Text Commentary
Full Time The match has reached full-time.90:00 +2:33
Jake Livermore concedes a free kick for a foul on John Eustace. Mark Yeates has an effort direct from the free kick, save by Carlo Cudicini.90:00 +1:10 Substitution
Substitution Roman Pavlyuchenko is brought on as a substitute for Emmanuel Adebayor.90:00 +0:14
The referee penalises Emmanuel Adebayor for handball. John Eustace takes the direct free kick.88:18
Free kick taken by Danny Rose.88:18 Booking
Booking The referee books Lee Hodson for unsporting behaviour.88:14
Free kick awarded for a foul by Lee Hodson on Aaron Lennon.86:10
Scott Parker takes a shot. Scott Loach makes a save.83:44
Mark Yeates delivers the ball, Younes Kaboul manages to make a clearance.82:19
A cross is delivered by Lee Hodson.81:10
Marvin Sordell produces a cross, Mark Yeates takes a shot. Michael Dawson gets a block in.80:24
The referee blows for offside. Indirect free kick taken by Scott Loach.80:05
Rafael Van der Vaart fouled by John Eustace, the ref awards a free kick. Free kick taken by Younes Kaboul.79:25 Substitution
Substitution Jermain Defoe leaves the field to be replaced by Steven Pienaar.79:11 Substitution
Substitution Joe Garner leaves the field to be replaced by Matthew Whichelow.77:46
Free kick awarded for an unfair challenge on Marvin Sordell by Younes Kaboul. Mark Yeates produces a cross, Nyron Nosworthy takes a shot. Carlo Cudicini makes a save. Prince Buaben takes a shot. Blocked by Kyle Walker.75:26
Corner taken by Mark Yeates from the right by-line, Headed effort from inside the area by Joe Garner misses to the left of the target.75:20
Marvin Sordell sends in a cross, clearance made by Michael Dawson.74:07
The ball is delivered by Troy Deeney, Effort on goal by Joe Garner from just inside the area clears the bar.73:33
Prince Buaben takes a shot. Save by Carlo Cudicini.
The referee blows for offside against Danny Rose. Scott Loach restarts play with the free kick.68:44
Corner taken right-footed by Mark Yeates from the left by-line, Header from inside the area by John Eustace strikes the post.68:35
Lee Hodson sends in a cross, save made by Carlo Cudicini.67:46
Troy Deeney has shot on goal from just outside the box which goes wide of the right-hand upright.67:04
Corner taken by Mark Yeates from the right by-line to the near post, clearance made by Danny Rose.66:52 Substitution
Substitution Mark Yeates joins the action as a substitute, replacing Sean Murray.66:37
The ball is swung over by Marvin Sordell, blocked by Younes Kaboul.65:18
Sean Murray crosses the ball, Carlo Cudicini makes a save.64:55
John Eustace sends in a cross, Michael Dawson makes a clearance.64:13
Rafael Van der Vaart takes a shot. Jermain Defoe crosses the ball, save by Scott Loach.63:41
Marvin Sordell takes a shot. Michael Dawson gets a block in.55:52
Centre by Kyle Walker, Lee Hodson makes a clearance. The ball is crossed by Danny Rose.53:11
Corner taken by Rafael Van der Vaart from the right by-line to the near post, clearance made by Marvin Sordell.52:38
The ball is delivered by Danny Rose, Kyle Walker takes a shot. Scott Loach makes a save.51:06
Free kick awarded for a foul by Prince Buaben on Jake Livermore. Strike on goal comes in from Rafael Van der Vaart from the free kick.50:17
Free kick awarded for a foul by Joe Garner on Scott Parker. Younes Kaboul restarts play with the free kick.48:38
Shot by Sean Murray from deep inside the penalty box hits the post. Troy Deeney has an effort at goal from deep inside the area missing to the right of the goal.47:42
Corner from the right by-line taken by Sean Murray, Shot from deep inside the area by John Eustace goes over the bar.47:28
Scott Parker challenges John Eustace unfairly and gives away a free kick. Adrian Mariappa takes the free kick.46:19
Effort on goal by Prince Buaben from outside the box goes harmlessly over the bar.45:01
The match restarts for the second half.45:01 Substitution
Substitution Aaron Lennon on for Luka Modric.45:00 +1:06 Half time
Half Time The ref blows to end the first period.43:06
Centre by Marvin Sordell, clearance made by Scott Parker.41:58
Assist by Kyle Walker.41:58 Goal scored
Goal! - Rafael Van der Vaart - Watford 0 - 1 Tottenham Rafael Van der Vaart finds the net with a goal from long distance to the bottom right corner of the goal. Watford 0-1 Tottenham.41:47
Lee Hodson crosses the ball.36:25
Unfair challenge on Danny Rose by Joe Garner results in a free kick. Younes Kaboul restarts play with the free kick.35:48
Michael Dawson fouled by Joe Garner, the ref awards a free kick. Michael Dawson takes the free kick.34:24
Shot from just outside the area by Prince Buaben misses to the right of the target.33:27
Prince Buaben takes a shot. Michael Dawson gets a block in.33:15
Marvin Sordell takes a shot. Michael Dawson gets a block in.30:13
Foul by Nyron Nosworthy on Danny Rose, free kick awarded. Rafael Van der Vaart crosses the ball in from the free kick, Lloyd Doyley manages to make a clearance.28:38
Jermain Defoe has a mishit shot. Save by Scott Loach.28:00
Unfair challenge on Luka Modric by Joe Garner results in a free kick. Danny Rose takes the direct free kick.25:34
Jake Livermore gives away a free kick for an unfair challenge on Joe Garner. Free kick crossed right-footed by Sean Murray, clearance made by Scott Parker.24:36
The ball is delivered by Troy Deeney, Joe Garner takes a shot. Save by Carlo Cudicini.23:46
A cross is delivered by Lloyd Doyley, Younes Kaboul makes a clearance. Lee Hodson has shot on goal from just outside the box which goes wide of the right-hand upright.22:45
The ball is sent over by Danny Rose, clearance made by Nyron Nosworthy.21:33
A cross is delivered by Lee Hodson.21:23
Troy Deeney crosses the ball, Younes Kaboul makes a clearance.15:26
Shot from outside the penalty box by Prince Buaben goes wide right of the goal.14:11
A cross is delivered by Troy Deeney, Shot from inside the six-yard box by Marvin Sordell goes wide right of the goal.11:45
A cross is delivered by Danny Rose, Adrian Mariappa manages to make a clearance.11:00
A cross is delivered by Kyle Walker, Nyron Nosworthy manages to make a clearance.9:55
Troy Deeney challenges Danny Rose unfairly and gives away a free kick. Danny Rose takes the direct free kick.8:18
The ball is sent over by Troy Deeney, clearance by Kyle Walker. John Eustace produces a right-footed shot from just outside the box that goes wide right of the target.7:31
Free kick awarded for a foul by Prince Buaben on Scott Parker. Direct free kick taken by Luka Modric.7:06
Sean Murray takes a shot. Carlo Cudicini makes a save.6:57
Centre by Lee Hodson, Kyle Walker manages to make a clearance.6:21
Free kick awarded for an unfair challenge on Rafael Van der Vaart by Adrian Mariappa. Direct free kick taken by Luka Modric.5:57
Joe Garner takes a shot. Blocked by Scott Parker.5:19
Marvin Sordell takes a shot. Save made by Carlo Cudicini.0:48
Troy Deeney produces a cross, Younes Kaboul manages to make a clearance.0:00
The game gets underway.
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