After Tamas Priskin had fired the home side into an early lead, Spurs equalised in first-half stoppage time with a Roman Pavlyuchenko penalty. Substitute Darren Bent then came off the bench to put the visitors within 180 minutes of another Wembley final.
A shock looked like it might be on the cards in the 13th minute when Watford were rewarded for their positive start with a goal of real quality.
It originated from a tremendous challenge by Jon Harley on Aaron Lennon, who broke forward before feeding Tommy Smith to his left.
The Hornets' top scorer took on and beat Vedran Corluka and got to the by-line before feeding Priskin in the penalty area. The Hungarian wasn't closed down by Jonathan Woodgate and Jermaine Jenas as he turned and fired a superb shot inside Heurelho Gomez's near post.
Tottenham, predictably, upped the pressure after that and Lennon forced a fine save out of Scott Loach with a curling shot, before Pavlyuchenko missed a great chance to equalise in the closing stages of the first half when he placed a shot wide of Loach, but then watched as it rebounded clear off the face of the crossbar.
However, the striker was given an opportunity to redeem himself in first-half stoppage time when Ross Jenkins tripped Jenas in the area, and the Russian duly converted from 12 yards.
The quality of the chances decreased after the break, although Spurs might have taken the lead when Woodgate's header from a corner was headed off the line by Lee Williamson.
That opportunity arrived after 75 minutes and within 60 seconds the visitors were ahead.
Jamie O'Hara fed Pavlyuchenko near the edge of the Hornets box and, as he sought to slide the ball through, it ricocheted across to the right side of the area where Bent took a touch before drilling a low shot inside Loach's near post.