OXFORD United were unable to make their dominance pay as Portsmouth hung on for a point in a stalemate at the Kassam Stadium.
The U's forced lengthy spells of pressure in both halves, but struggled to create clear chances.
Their best moment arguably came just after the hour, when Danny Rose fired across the face of goal from a narrow angle, but goalkeeper Trevor Carson was frustratingly severely not tested all afternoon.
Portsmouth were dogged and their attacking threat was well shackled by the hosts, with U's goalkeeper Ryan Clarke having less to do than even his opposite number.
But the visitors were ultimately far happier with the result, as United drew a blank in their first game of 2014.
New signing Nicky Wroe came straight into the hosts' starting line-up as one of two changes to the starting line-up.
The other saw Michael Raynes promoted, as Chris Wilder went with a 4-3-3 system which saw Mullins screening the back four, just behind Wroe and Danny Rose.
Portsmouth, backed by 2,300 noisy fans, were the first to get a sight of goal, when Patrick Agyemang dragged an early effort wide.
Aside from a Jed Wallace effort which sailed well over, it was almost as good as it got for them going forward in the first half.
United took time to establish themselves as an attacking force.
Deane Smalley, who led the line as James Constable dropped to the bench, had limited impact getting on the end of aerial balls forward. Flanking him were Sean Rigg and Ryan Williams, who played in a more narrow role than usual but struggled to stretch the Pompey back four.
Gradually the home side began to make headway and in the final 15 minutes of the half the visitors were put under pressure.
Tom Newey, who was a constant outlet on the left flank, came as close as anyone to breaking the deadlock before the break when he leapt to head Danny Rose's 32nd-minute corner just over.
David Hunt curled a free-kick just off target and the full back then crossed for Raynes, whose header was blocked and scrambled behind by an increasingly uncertain Portsmouth defence.
The visitors started the second half brightly, forcing three quick corners. From the last of the trio Nicky Shorey almost curled his delivery straight in, but the ball slid past the far post.
United quickly restored their control on the game, forcing a couple of corners themselves.
Constable replaced an out of sorts Smalley on the hour mark and quickly helped to create a good chance for Rose. The striker's cross rebounded kindly for Rose, who could only fire across the face of goal from an acute angle.
United were unquestionably on top, but they relied on their full backs to provide the width as Williams, and particularly Rigg, came inside.
It made it difficult to get in behind Portsmouth's defence and when a chance did arrive, Carson in goal was not tested.
Hunt volleyed wide when Newey's cross was only half-cleared before, with 12 minutes left, Williams came close to breaking the deadlock.
Wroe's ball forward to Constable was knocked down into the winger's path but although he caught it sweetly on his left foot, the 20-yard volley fizzed just over.
Portsmouth looked poor, but came close to pinching a goal late on. Wallace's inswinging free-kick found Joe Devera free at the back post, but the centre back missed the ball and ended up in the net himself.
Mullins had the final effort when, deep into stoppage time, he got hold of a volley from range which was well struck but straight at Carson.
It was the final act of a game which was an improvement on their 2-0 defeat to Scunthorpe in the previous game, but it remained a wasted opportunity.
Oxford Utd: Clarke, Hunt, Raynes, Wright, Newey, Mullins, Wroe, Rose, Williams, Smalley (Constable 61), Rigg.
Unused subs: Henly, Davies, Marsh, Long, Bevans, Ruffels.
Portsmouth: Carson, Alfei, Devera, Bradley, Painter, Ferry, Racon (Padovani 77), Shorey, Wallace, Taylor (Ertl 90), Agyemang (Bird 72).
Unused subs: Smith, N'Gala, Connolly, Whatmough.
Attendance: 8,443 (2,365 visitors).
Referee: Mark Heywood (Cheshire).