Nathan Delfouneso came off the bench to score his first Premier League goal and keep Aston Villa's faint hopes of a top-four finish alive at Portsmouth.
It was no more than Villa deserved, the young striker prodding home on 82 minutes from Emile Heskey's flick-on.
But it was still a little harsh on the hosts after another brave display, kick-started by Michael Brown's curler.
John Carew drilled the leveller but also missed a penalty among a glut of chances, before Villa snatched victory.
And it could prove a crucial intervention from 19-year-old Delfouneso, his winner pushing Villa up to sixth spot, two points ahead of Liverpool, four behind Manchester City and six off fourth-placed Tottenham.
Tottenham's defeat of Chelsea on Saturday had prompted manager Martin O'Neill to reason before kick-off that his Villa side needed to win all of their last five games to have a chance of grabbing the final Champions League spot.
And despite a mixture of good goalkeeping, poor finishing and some strange decisions from referee Lee Probert, the visitors eventually secured a potentially priceless three points with four matches left in their season.
Portsmouth played their part in an entertaining and remarkably open tie, though.
Boosted by the return of a number of first-teamers, the hosts played with enviable freedom on the counter attack, while Villa pushed and probed with width and pace throughout.
As a result, chances for both sides were aplenty.
Villa keeper Brad Friedel did well to deny Kevin-Prince Boateng's snap-shot early on, Kanu seeing his follow-up blocked, while the American also tipped a Boateng free-kick wide and charged down John Utaka well to deny the Nigerian through on goal.
He could do little about Portsmouth's opener, though, Boateng cleverly stepping over Anthony Vanden Borre's cross and Brown curling a fine shot into the far corner for his first goal for the club.
At the other end, Carew somehow fired straight at James with the goal gaping, while Marc Wilson was lucky to see his attempted clearance from Ashley Young's cross rebound back off the post.
And between those two chances, Villa deservedly drew level. Carew was the goalscorer to make amends for his earlier miss, the Norwegian - onside - dashing onto Stephen Warnock's ball over the top, turning inside Wilson and thrashing into the roof of the net.
606: DEBATE
Can Aston Villa finish fourth?
Probert, the same official that denied Bolton two strong penalty appeals against Chelsea in midweek, then took centre stage.
First he waved away Portsmouth's shouts for a spot-kick when Richard Dunne looked to have felled Boateng clumsily in the box, and then bizarrely did likewise to Villa's appeals despite Vanden Borre clearly hauling Agbonlahor to the ground in the home penalty area.
He finally gave in on 40 minutes after Papa Bouba Diop tripped Carew, but James responded with a smart save from the striker's penalty.
It did not dent Villa's ambition after the break, though, as the visitors continued to create chance after chance.
Agbonlahor will kick himself for heading straight at James from six yards out from Young's fine cross, while James Collins headed over unmarked soon after and Milner saw a long-ranger beaten away by the keeper.
And there were two further penalty appeals waved away as well, firstly when Young appeared to be tripped by Vanden Borre and the second when Stylian Petrov went over James's sprawling challenge in the box.
For all their pressure, though, it looked like Villa would have to settle for a point and possibly downgrade their ambitions of a top-six finish as well as the minutes ticked down with Portsmouth looking increasingly comfortable.
But Delfouneso had other ideas, the youngster taking only two minutes to fire home the winner after fellow substitute Heskey had flicked on Milner's cross.
Portsmouth boss Avram Grant:"Of course we wanted to win the game today, but first and foremost we wanted to play with pride, put on a good performance, and be professional and we did that.
"I am very happy with the boys even though we lost. We have so many players unavailable and yet we can play like that against a very good team - that pleases me.
"Despite everything that has happened with this team - and it is not easy - we continue to fight. I am very proud of them."
Aston Villa manager Martin O'Neill:"I'm delighted for Nathan (Delfouneso). I've been saying this for some considerable time, but he's going to be a proper player and if he keeps his feet on the ground, he's got a real proper chance.
"Now, to give ourselves any chance (of finishing fourth) we have to win all our games. Today would have been disappointing, considering the amount of possession we had away from home, not to have taken all three points.
"I'm delighted we've done that and we go to Hull on Wednesday and we're still full of hope."
It was no more than Villa deserved, the young striker prodding home on 82 minutes from Emile Heskey's flick-on.
But it was still a little harsh on the hosts after another brave display, kick-started by Michael Brown's curler.
John Carew drilled the leveller but also missed a penalty among a glut of chances, before Villa snatched victory.
And it could prove a crucial intervention from 19-year-old Delfouneso, his winner pushing Villa up to sixth spot, two points ahead of Liverpool, four behind Manchester City and six off fourth-placed Tottenham.
Tottenham's defeat of Chelsea on Saturday had prompted manager Martin O'Neill to reason before kick-off that his Villa side needed to win all of their last five games to have a chance of grabbing the final Champions League spot.
And despite a mixture of good goalkeeping, poor finishing and some strange decisions from referee Lee Probert, the visitors eventually secured a potentially priceless three points with four matches left in their season.
Portsmouth played their part in an entertaining and remarkably open tie, though.
Boosted by the return of a number of first-teamers, the hosts played with enviable freedom on the counter attack, while Villa pushed and probed with width and pace throughout.
As a result, chances for both sides were aplenty.
Villa keeper Brad Friedel did well to deny Kevin-Prince Boateng's snap-shot early on, Kanu seeing his follow-up blocked, while the American also tipped a Boateng free-kick wide and charged down John Utaka well to deny the Nigerian through on goal.
He could do little about Portsmouth's opener, though, Boateng cleverly stepping over Anthony Vanden Borre's cross and Brown curling a fine shot into the far corner for his first goal for the club.
At the other end, Carew somehow fired straight at James with the goal gaping, while Marc Wilson was lucky to see his attempted clearance from Ashley Young's cross rebound back off the post.
And between those two chances, Villa deservedly drew level. Carew was the goalscorer to make amends for his earlier miss, the Norwegian - onside - dashing onto Stephen Warnock's ball over the top, turning inside Wilson and thrashing into the roof of the net.
606: DEBATE
Can Aston Villa finish fourth?
Probert, the same official that denied Bolton two strong penalty appeals against Chelsea in midweek, then took centre stage.
First he waved away Portsmouth's shouts for a spot-kick when Richard Dunne looked to have felled Boateng clumsily in the box, and then bizarrely did likewise to Villa's appeals despite Vanden Borre clearly hauling Agbonlahor to the ground in the home penalty area.
He finally gave in on 40 minutes after Papa Bouba Diop tripped Carew, but James responded with a smart save from the striker's penalty.
It did not dent Villa's ambition after the break, though, as the visitors continued to create chance after chance.
Agbonlahor will kick himself for heading straight at James from six yards out from Young's fine cross, while James Collins headed over unmarked soon after and Milner saw a long-ranger beaten away by the keeper.
And there were two further penalty appeals waved away as well, firstly when Young appeared to be tripped by Vanden Borre and the second when Stylian Petrov went over James's sprawling challenge in the box.
For all their pressure, though, it looked like Villa would have to settle for a point and possibly downgrade their ambitions of a top-six finish as well as the minutes ticked down with Portsmouth looking increasingly comfortable.
But Delfouneso had other ideas, the youngster taking only two minutes to fire home the winner after fellow substitute Heskey had flicked on Milner's cross.
Portsmouth boss Avram Grant:"Of course we wanted to win the game today, but first and foremost we wanted to play with pride, put on a good performance, and be professional and we did that.
"I am very happy with the boys even though we lost. We have so many players unavailable and yet we can play like that against a very good team - that pleases me.
"Despite everything that has happened with this team - and it is not easy - we continue to fight. I am very proud of them."
Aston Villa manager Martin O'Neill:"I'm delighted for Nathan (Delfouneso). I've been saying this for some considerable time, but he's going to be a proper player and if he keeps his feet on the ground, he's got a real proper chance.
"Now, to give ourselves any chance (of finishing fourth) we have to win all our games. Today would have been disappointing, considering the amount of possession we had away from home, not to have taken all three points.
"I'm delighted we've done that and we go to Hull on Wednesday and we're still full of hope."
