Portsmouth 1 (Agyemang 24) Oxford United 4 (Smalley 34 & 71, Potter 38 & 64)

The visitors fell behind to Patrick Agyemang's 24th-minute header, but from then on the game belonged to them.

Smalley and Potter's quick double turned the game on its head before half-time.

Portsmouth's irritation boiled over when skipper Johnny Ertl received a straight red card for an elbow on Danny Rose just before the hour.

And United made the most of the man advantage, with Potter and Smalley helping themselves to further goals.

The second half strikes came in front of an increasingly delirious away end, who relished the occasion.

There were no surprises in Chris Wilder's starting line-up, with Dave Kitson, David Hunt and Tom Newey all making their competitive debuts for the club.

A packed Fratton Park shook in the build-up as both sets of fans made themselves heard.

Portsmouth wasted little time in seizing the initiative and quickly found their stride.

The home side controlled the majority of possession in a frenetic opening ten minutes, as United battled to keep them at arm's length.

It took 20 minutes for a chance to arrive, though, when Agyemang gave a warning of his aerial prowess by heading over at a corner.

The visitors had barely strung two passes together in the Portsmouth half, but a Rose free-kick gave them a launch pad midway through the half.

His delivery was flicked by Kitson for Johnny Mullins, whose volley flew wide, just out of Smalley's reach.

United had just begun to settle and quieten the crowd when Portsmouth opened the scoring.

Andy Barcham found a yard of space wide on the left and swung in a cross which Agyemang powerfully headed past Ryan Clarke.

Pompey were buoyant, but United responded and launched a stunning counter-attack with two goals in four minutes.

On 34 minutes Potter was given space for the first time and he made the most of it.

The winger brushed past Dan Butler and advanced, crossing low for Smalley, who spun and finished past John Sullivan in the Portsmouth goal.

It sparked wild celebrations from the 2,500 travelling fans at the other end, but it got even better four minutes later.

Smalley chased a hopeful ball from Hunt and outmuscled Yassin Moutaouakil. He picked out Sean Rigg, who squared perfectly to find Potter's late run and the winger coolly converted from 12 yards to put United ahead.

The visitors safely negotiated the final seven minutes of the half and regrouped at the break.

Portsmouth went in search of a way back into the game when play restarted, but they were again hampered by United's work rate.

Frustration began to show and it provoked an error of judgement just before the hour mark.

Rose went down in a heap after an aerial challenge with Ertl and referee Gavin Ward had no hesitation in producing a red card.

If that made Portsmouth's task of getting back into the game hard, it quickly became even more difficult.

Rigg and Smalley combined again to tee up Potter, who came in from the right and from the edge of the box threaded a low left foot shot past Sullivan at his near post.

Portsmouth began to disintegrate and the U's continued to go for the jugular.

On 71 minutes Potter again caused problems down the right, crossing for Smalley to steer a shot into the net.

Former United striker Tom Craddock had a chance to reduce the arrears when Clarke spilled Barcham's shot, but he could not adjust his feet in time and skewed a shot wide.

It was the final clear opportunity for Portsmouth, who knew they were a beaten side.

United move on to Charlton Athletic in the Capital One Cup on Tuesday night.

Portsmouth: Sullivan, Moutaouakil, Devera, Bradley, Butler (Holmes 62), Wallace (Padovani 69), Ertl, Ferry, Barcham, Agyemang (Craddock 69), Connolly.

Subs not used: Smith, East, N'Gala, Bird.

Oxford: Clarke, Hunt, Mullins, Wright, Newey, Potter, Whing, Rose, Rigg (Davies 88), Smalley (Constable 73), Kitson (Hall 82).

Subs not used: Crocombe, Raynes, Williams, Ruffels.

Referee: Gavin Ward.

Attendance: 18,181 (2,500 visitors).