Harry Redknapp took a break from denying two counts of cheating the public revenue to watch his Tottenham Hotspur team progress rather fortuitously to the FA Cup fifth round.

Redknapp, who has a further five days at least at Southwark crown court while his case continues, had been unable to take training all week and there was evidence in a lax display that his squad may have missed his exhortations at their Spurs Lodge base in the build-up to this tie.

He acknowledged this had been a disappointing performance by joking afterwards: "I thought we were fantastic, the football was mind-blowing, I've never been so excited watching us play. It was just unbelievable – how we won that game we'll never know. They say it's better to be lucky than good sometimes and we were lucky today for sure. Amazing, we just never played. If they [Watford] played as well as that every game, they'd be top of the league."

Redknapp could find an augury here that was similar to when he led Portsmouth to FA Cup glory in 2008. "We went to Preston and I've never seen a team get battered like it," he said of that fifth-round tie. "They missed a penalty with four minutes to go at nil-nil. David James saved it, we go up the other end – I think the first time we got in their half – and we scrambled a fluke of a goal from a corner [from a Darren Carter own-goal] and we go on and win the Cup."

Watford had written the tale of the first half, with Spurs' contribution barely a footnote until Rafael van der Vaart scored. Taking the ball from Kyle Walker inside home territory, the Dutchman wandered towards goal and from 30 yards unloaded a shot that bounced over the prone Scott Loach, who will be disappointed.

Marvin Sordell had given Spurs an early warning as to what kind of evening to expect but troubled the visiting defence far less than the Sean Murray attempt moments later that had Carlo Cudicini unable to hold on to the ball before it was scrambled away to safety.

Redknapp had fielded a strong XI that included Luka Modric, Emmanuel Adebayor, Van der Vaart, Jermain Defoe, Michael Dawson and Scott Parker. They woke up momentarily after 13 minutes when Adebayor shrugged off Lee Hodson, the home right-back, and bounced a pass into Walker on the opposing flank who floated in a cross that the Togolese forward was crowded out from finishing by a clutch of Watford defenders.

This was about as good as it got for Spurs until Van der Vaart's intervention. The hosts continued to worry Cudicini: Troy Deeney moved down the right and pinged in a cross that Sordell put wide; then a midfield mix-up allowed Prince Buaben to send an effort narrowly wide.

There was more. Joe Garner, playing off Sordell in Watford's attack, dumped Van der Vaart to ground, then played in a pass to his strike partner that Walker had to clean up.

In a frantic opening to the second half John Eustace had the first opportunity to level when a corner fell to him but the home captain blasted over from close range. Murray was then unlucky to hit Cudicini's left post, as was Eustace who subsequently did the same with a header. "We were good value for our performance, for sure," said Sean Dyche, the Watford manager. He was correct. But it is Spurs who await Sunday's draw.