The home side were awarded an early penalty for a handball, but with four minutes on the clock, Jamie Cumming guessed the right way and saved from Elias Sorensen.

Pompey took the lead shortly before an hour had been played as former United winger Josh Murphy came back to haunt his old club when he crossed for Mark O’Mahony to glance a header in.

Less than quarter of an hour later though, Sibley, who had only just come on as a substitute, found the back of the net from the edge of the box.

Des Buckingham made two changes to the side which earned a draw in the pulsating clash at Luton Town, with one of those enforced.

U’s skipper Elliott Moore missed out with the knee problem which forced him off in stoppage time at the Hatters, and Sam Long came into the side as captain.

The other switch was on the right wing, as Kyle Edwards replaced Owen Dale, with Edwards making his first start since the goalless draw at home to Bolton Wanderers on November 28.

With just three minutes on the clock, the home side were awarded a penalty for a soft-looking handball by Will Vaulks, but Cumming came to the rescue and dived low to his left and pushed away the spot kick from Sorensen.

United grew into the game and had their first sniff of goal just before quarter of an hour had been played.

Ruben Rodrigues got to the right-hand byline and crossed for Mark Harris in the six-yard box, however the striker kicked the ball against his left foot and it bounced out of play for a goal kick.

Sorensen, looking to make amends for his saved penalty, then smashed over when a corner was headed clear to him on the edge of the penalty area.

With 20 minutes played, Ciaron Brown sent a clever left-sided cross over the top of the Pompey backline for Rodrigues, but the midfielder couldn’t get the ball under control and he shot wide with his weaker left foot while off balance.

Idris El Mizouni then brilliantly squirmed his way past a huddle of Portsmouth players, bursting through the midfield, before making a run for the penalty box.

He looked to place his shot into the far corner, but Nicolas Schmid got down low and tipped the ball round the post.

Tyler Goodrham did well to win the ball on the left, but Rodrigues couldn’t steer the ball home while on the stretch, while Long headed off target from a Will Vaulks corner.

Harris worked himself space 20 yards out, but drilled his strike down the centre of the goal, allowing Schmid to collect it with ease, and in the final bit of goalmouth action during the first half, Connor Ogilvie flicked an effort at Cumming from close range, with the goalkeeper saving at his near post.

Ben Nelson appeared to pick up a muscular knock towards the end of the opening 45 minutes, and at half-time, he was withdrawn, as Greg Leigh came on in his place.

The change saw Brown move inside from left back to partner Long at centre half, while Leigh played on the left side of the back four.

Pompey took the lead just before the hour mark, as Murphy crossed from the left for Brighton & Hove Albion loanee O’Mahony, who glanced the ball into the far corner.

Buckingham made an immediate change, introducing Siriki Dembele for Edwards, with Dembele going down the left, as Goodrham switched to the right.

U’s head coach Buckingham then made a double substitution nine minutes later, as Sibley and Dane Scarlett replaced Rodrigues and Harris respectively.

Sibley had an immediate impact, carrying the ball into space before striking from the edge of the area, with the ball taking a nick before nestling into the corner.

Long then produced heroics at the other end when he blocked an O’Mahony shot from the centre of the box, while Ogilvie and most of Fratton Park thought the defender had restored the Pompey lead, but his header from a corner crashed off the post.

The game at this point became frantically open, and Dembele raced through the centre of the pitch before firing off target.

Murphy then saw a fizzing low strike deflect the other side of the near post, while Ogilvie headed over in stoppage time, as the sides shared the points.

Portsmouth (3-4-2-1): Schmid; Poole, McIntyre, Ogilvie; Williams (Swanson 84), Potts, Pack, Murphy; Sorensen (Lane 90+2), Saydee (Ritchie 84); O’Mahony (Blair 79).

Unused subs: Archer, Towler, Silvera, Dozzell, Kamara.

Booked: None.

Oxford United (4-3-3): Cumming; Kioso, Long, Nelson (Leigh 46), Brown; Rodrigues (Sibley 69), Vaulks, El Mizouni; Edwards (Dembele 60), Harris (Scarlett 69), Goodrham.

Unused subs: Ingram, ter Avest, Dale, Ebiowei, Goodwin.

Booked: Kioso.

Referee: Lewis Smith (Lancashire)

Attendance: 20,125 (1,936 away)