Roman Pavlyuchenko is congratulated after his penalty
Darren Bent's late strike kept Carling Cup holders Spurs on course for Wembley with a quarter-final win at Watford.
Tamas Priskin gave the Championship strugglers a shock lead with a shot on the turn after 13 minutes.
Spurs created a host of chances before Roman Pavlyuchenko scored from the spot in first-half injury time after Ross Jenkins had fouled Jermaine Jenas.
Substitute Bent then beat keeper Scott Loach at his near post with 14 minutes left to end Watford's resistance.
The deserved win put Harry Redknapp's side into the last four, along with Burnley, Derby County and Manchester United.
Watford were spirited, despite being outclassed for lengthy spells, and new boss Brendan Rodgers will take heart from his side's resilience as he starts the job of keeping his side in the Championship.
The Hornets needed a fast start to put their Premier League opponents under pressure - and they got it in the shape of that early goal from Priskin.
Tommy Smith was the creator with a cross from the left flank, giving the Hungarian the chance to turn and beat Heurelho Gomes with a low, left-foot shot.
It was the catalyst for Spurs to lay siege to the Watford goal for the rest of the half, but Rodgers' side almost survived until the interval.
Watford keeper Loach saved brilliantly from Aaron Lennon's effort with the outside of his foot, and then did well to deny Jenas.
Lennon suffered at the hands of Loach again, before Pavlyuchenko broke through and curled a shot against the bar two minutes before the interval.
Just as it looked as if Watford would reach half-time with their lead intact, they conceded a penalty deep into stoppage time.
Jenkins tripped Jenas as he touched the ball away from the Watford man, with referee Phil Dowd ignoring the protests to point to the spot.
Pavlyuchenko slid his penalty underneath the diving Loach, who was frustrated that he was not able to keep the Russian's effort out.
Spurs failed to capitalise on the goal after the re-start, even though Watford lost influential defender Jay DeMerit through injury, with Lloyd Doyley coming on as replacement.
Redknapp made an attacking change after 64 minutes, with Bent replacing Fraizer Campbell, who had been a subdued figure.
Jenas and Jamie O'Hara tested Loach, but the concerted pressure of the first period had not been applied by Spurs after the break.
Benoit Assou-Ekotto almost created an opening for Bent with 18 minutes left, firing in a low cross that just evaded the England striker at the far post.
Lee Williamson then scrambled Jonathan Woodgate's header off the line before Bent gave Spurs a deserved lead after 76 minutes.
It was a nightmare moment for the previously impressive Loach, who allowed Bent's angled shot to squirm in at his near post.
Watford cast caution aside in a bid to snatch an equaliser, and substitute Will Hoskins tested Gomes with a low shot in the dying moments - but Spurs survived to set their sights on another Wembley date.
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