Pompey’s play-off contest with Oxford is finely balanced after the sides drew 1-1 in the first leg at Fratton Park.
The hosts made the stronger start and deservedly took the lead through a low finish from Ronan Curtis.
But Marcus Browne grabbed a United leveller just before the interval and neither side were able to find a winner in the second half.
Kenny Jackett made four changes from the side that drew with Fleetwood all the way back in March.
Lee Brown wore the captain’s armband, with regular skipper Tom Naylor named on an extended nine-man bench.
There were also returns for Christian Burgess, Marcus Harness and a fit-again Bryn Morris, as James Bolton, Ryan Williams and Steve Seddon made way.
A blustery Fratton Park was virtually empty, with the Covid-19 pandemic necessitating games taking place behind closed doors.
But there were still more than 1,000 fans present in the form of cardboard cut-outs, with 2003 promotion hero Svetoslav Todorov, chairman Michael Eisner and the late John Jenkins MBE among them.
Before kick-off, there was a minute of applause to celebrate the NHS and other key workers, while all players then took the knee in a powerful statement against racism.
The hosts controlled possession for much of the first half and Ellis Harrison had an early header blocked from a corner.
And the deadlock was almost broken moments later, courtesy of a drilled Curtis effort that struck the base of the post.
Both sides were clearly up for the challenge, with several meaty challenge flying in and Oxford boss Karl Robinson constantly in the ear of a beleaguered fourth official.
Neither keeper was really being put to the test and a Harness curler was blocked, while it took a diving Josh Ruffels header to deal with one Curtis delivery.
After a brief break for drinks, Cameron Brannagan and Cameron McGeehan were both booked following a spat in the middle of the pitch.
Burgess then thought he should have won a penalty after appearing to be tugged back by Rob Dickie, but appeals were dismissed by referee Gavin Ward.
Pompey’s pressure did pay off on 32 minutes, though, as CURTIS advanced and hit a low shot that crept past former Fratton stopper Simon Eastwood and nestled in the net.
That seemed to spark the visitors into life and Browne dragged an effort past the post, although Alex Bass appeared to have it covered.
The Blues could not keep their opponents at bay, though, and Browne did well to burst forward and find the net.
But Pompey did finish the half with a flourish and visiting players had to fling themselves in front of a flurry of shots in quick succession.
Half Time: Pompey 1 Oxford 1
There was a slow start to the second period in terms of chances, although Harrison felt a challenge on him inside the box warranted a penalty.
Jackett tried to liven up proceedings with a double switch just past the hour mark, as Ross McCrorie and Andy Cannon made way for Bolton and Williams.
But it was Oxford who were next to have a shot, albeit a long-range James Henry attempt that Bass held comfortably.
Williams soon showed some neat trickery, although his low cross was cut out before it could reach Harrison in the centre.
The Blues then delivered a couple of fine corners – first from Brown and then Harness – that were just about dealt with.
Jackett made his third change of the evening on 73 minutes and it was a straight substitution up front, as Harrison made way for John Marquis.
The hosts were starting to build some momentum and Brown’s effort was deflected into the side netting, while the skipper then did well to halt Browne’s break at the other end.
And Burgess went close to restoring the lead when he met Brown’s free-kick, only for Eastwood to get down and make the save.
Curtis was furious not to be awarded a late penalty after going down under a challenge from Dan Agyei, but again Ward said no.
Pompey pushed forward in search of a lead to take to the Kassam Stadium on Monday, but had to settle for a draw.
Pompey (4-2-3-1): Bass; McCrorie (Bolton 62), Burgess, Raggett, Brown (c); McGeehan, Morris; Harness, Cannon (Williams 62), Curtis; Harrison (Marquis 73)
Goals: Curtis 32
Subs not used: MacGillivray, Downing, Seddon, Naylor, Evans, Hawkins
Oxford (4-1-4-1): Eastwood; Long, Dickie (c), Moore, Ruffels; Gorrin; Henry, Woodburn (Sykes 55), Brannagan (Forde 81), Browne (Agyei 81); Taylor (Mackie 73)
Goals: Browne 43
Booked: Brannagan, Gorrin
Subs not used: Stevens, Atkinson, Mousinho, Kelly, Hanson
Referee: Gavin Ward