OXFORD United slipped to the bottom of the fledgling Sky Bet League Two table after failing to turn another promising performance into a result as they fell to defeat at home to Portsmouth.
Craig Westcarr pounced on a defensive error ten minutes from time to inflict a fourth successive defeat in the league.
United's best moments were all before the break and they came closest on the stroke of half-time, when Danny Hylton hit the post and then had a strong penalty claim turned down.
But the hosts lost their way after the hour mark, allowing Portsmouth into the game - and the visitors made the most of their opportunity.
United were unchanged from Tuesday's 1-0 defeat at Morecambe and made a very bright start.
Just ten seconds were on the clock when Hylton was fouled 20 yards out.
Joe Riley stepped up and ripped a shot towards the top corner, which goalkeeper Paul Jones tipped over.
From the resulting corner Carlton Morris had two efforts blocked, the second of which by Jones on the line.
Portsmouth, backed by more than 2,000 travelling fans, arrived unbeaten in all competitions and should have scored on nine minutes.
A long ball was only headed upwards by Johnny Mullins, allowing the hosts to get forward. Josh Ruffels was unable to cut out a clever through ball for Nigel Atangana, who let the home side off the hook by skidding a shot wide from just 15 yards.
When the game settled down, United assumed their increasingly familiar style of patient, possession football.
Clear chances were hard to come by for much of the half, but Atangana came close to putting through his own net in the 20th minute.
The backtracking midfielder inadvertently turned Danny Rose's corner goalwards and was relieved to see the ball was cleared by the man on the post.
United finished the half strongly, as they put Portsmouth under increasing pressure.
Ricky Holmes was fortunate to stay on the field when, already on a booking, he dived in on Junior Brown.
The midfielder escaped with only a final warning from referee Darren Bond.
Both sides had good chances to score late in the half.
First, Portsmouth forced an opening when Miles Storey got free in the box, forcing a smart save from George Long.
At the other end, United did everything but score after a 44th-minute corner caused chaos in the area.
Hylton had three efforts in quick succession, with the second hitting the post and the third deflected over by James Dunne.
The striker thought he had won a penalty on the stroke of half-time when he was felled after a smart turn.
Just as at Morecambe on Tuesday night, the referee waved away the protests, but unlike in midweek Hylton was not booked.
United were furious and swarmed around Bond when he blew the whistle for half-time seconds later.
Hylton also had the opening chance of the second half, when he headed Riley's corner on target, but Jones gathered comfortably.
Chances dried up after that, as United appeared to run out of steam after the hour mark.
Portsmouth, who probably could not believe their luck to still be level, took up the initiative and grew in confidence.
They began to ask questions of the United defence for the first time and although little was created, the momentum had shifted.
And with ten minutes remaining, the stalemate was broken.
Nickey Shorey got some space out on the left to whip in a cross which United failed to deal with, allowing Westcarr to bundle into the net from close range.
An equaliser almost arrived within two minutes.
Brown burrowed into the area and got away a shot on the turn which was arrowing towards the bottom corner before Jones got fingertips on it to divert it wide.
The winger had a better chance three minutes from time, but he headed well over from six yards.
It was the final opening as Portsmouth saw out five minutes of stoppage time to leave United pointless.
Oxford Utd: G Long, Riley, Mullins, Wright, Newey, Collins, Ruffels (Potter 84), Rose (O'Dowda 73), Hylton, Brown, Morris (Jakubiak 84).
Unused subs: Crocombe, Hunt, Raynes, Meades.
Portsmouth: Jones, Devera, Robinson, Chorley, Holmes (Wallace ht), Hollands, Dunne, Atangana, Shorey, Westcarr (Whatmough 86), Storey (Taylor 64).
Unused subs: Brezovan, Butler, Awford, Barcham.
Referee: Darren Bond (Lancashire).
Attendance: 6,852 (2,136 visitors).