Pompey reopened the Premiership title race by overcoming United 2-1 less than 48 hours earlier, but they were no match for the bottom-of-the-table Hornets, who earned only their fourth league win of the campaign thanks to Hameur Bouazza's penalty and further strikes from Gavin Mahon, Tamas Priskin and the irresistible Bouazza.

Matthew Taylor had given the visitors a 16th-minute lead, but Watford had edged in front by half-time and their second-half display was excellent, the only blight on it an 81st-minute consolation from Pompey substitute Arnold Mvuemba.

The outcome might have been different had Kanu scored rather than hit the crossbar in the 58th minute, but Watford often overran Portsmouth after the interval and deserved their win fully.

Clarke Carlisle made his first appearance of the season for Watford after missing most of the campaign with thigh problems, although he did regain match-fitness during a recent loan spell at Luton.

Harry Redknapp's side soon found their stride and took the lead through a wonderful strike from Taylor after a quarter of an hour.

The recalled Priskin gave the ball away to Sean Davis, who transferred the ball to Taylor and the midfielder was given time and space by Jay DeMerit to curl a precise right-footed shot into Ben Foster's bottom-left corner from 25 yards.

The sense of resignation among the home fans was palpable as the ball hit the net, as they anticipated an eighth home league defeat of the season.

It was difficult to see how Watford would find the means to break down a solid-looking Portsmouth rearguard, but Priskin made up for his earlier mistake by winning the penalty which led to the equaliser.

In the 27th minute, the Hungary international deceived Djimi Traore with a clever turn and burst into the box, only for Traore to trip him.

From the spot-kick, Bouazza confidently sent David James the wrong way to ensure the England goalkeeper would have to wait a little longer for his Premiership record-breaking 142nd clean sheet.

The first half moved towards its conclusion with few chances for either side, but the Hornets seized the lead thanks to Mahon's superbly-struck effort in stoppage time.

Tommy Smith's cross was punched by James only as far as Mahon, who blasted the ball into the roof of the net from 20 yards for his first goal of the campaign.

Watford carried their momentum from the first half into the second and scored one of their best goals of the season in the 51st minute.

Bouazza collected the ball inside his own half and burst into Pompey territory before feeding the ball to his left, where Steven Kabba had time and space to pick out Priskin at the far post, who guided his close-range volley beyond James and into the bottom-left corner.

After scoring his first in the Premiership for the Hornets Priskin ran to the Watford bench, where he was embraced by the Hornets' injured striker Marlon King.

Only Pompey will know how they failed to halve the deficit in the 58th minute.

Kanu showed wonderful close control to evade three Watford defenders inside the box before crashing a shot against the crossbar.

Foster tried to claim the ball but collided with Carlisle, allowing Gary O'Neil to fire in a shot which Adrian Mariappa cleared off the line, the ball rebounding off Kanu and behind for a goal-kick.

But Watford, playing with a freedom they have rarely displayed this season, scored their fourth in the 72nd minute, and it was fitting that the excellent Bouazza should claim it.

Collecting Kabba's fine flighted return pass, Bouazza drove his first shot against James but took advantage of a favourable rebound to drill the ball home from six yards, leaving the delighted home fans to chant `Bring on United'.

In the 80th minute, Redknapp replaced Davis with Mvuemba, and the switch paid immediate dividends when the Frenchman controlled a cross from O'Neil and volleyed in from 12 yards to pull a goal back for his team.

Afterwards goal hero Mahon, whose side are still bottom of the Premiership despite their form in the cup, admitted: "We needed that, especially after a woeful performance against Middlesbrough.

"The fans have been waiting for that and it's good we delivered before the end of the season.

"We needed a big performance going into the semi-final and we now need a good week's training going into that game."

Watford boss Adrian Boothroyd was also looking towards the clash with United but praised his team for their perseverance after a tough season in the top flight.

"I'm delighted for a lot of reasons," he said.

"The main reason being that when a team takes as many knocks as we have taken it takes a lot of character to come back.

"There's a fear as a manager of 'can we come again?'

"We've got a semi-final on Saturday and after a woeful performance against Middlesbrough it was important (to win).

"I'm a players` manager and I think they need to develop and the players need to push themselves."