Grant's words at half-time had a visible effect on the home side as, they were far more competitive in the second half as a result of an increased tempo to their play.
Rarely were they able to translate promising situations into goalscoring opportunities and the sense that Villa need only up their game to end the tie was realised with 15 minutes remaining.
Petrov found Young in space on the Villa right and his cross beat the gathered players in the six yard box before being met by Downing, whose header crept in despite the efforts of Pompey keeper Begovic.
It was a fitting reward for a player who has endured a frustrating four months as a Villa player since his summer move from Middlesbrough because of a broken foot.
Grant immediately introduced Piquionne in an attempt to get back into the game and he almost did so with his first touch but Guzan saved well from the Frenchman's header.
Minutes later, another Portsmouth substitute Kanu gave the home side hope of a late comeback.
Younes Kaboul sent a long ball forward which was chested down by Hermann Hreidarsson for the Nigerian striker to slot in past Guzan.
However, Villa were not to be denied what would be deserved victory and within two minutes their two goal cushion was restored and their semi-final place booked.
Gabriel Agbonlahor beat Kaboul on the right and provided a cross that beat Hreidarsson and found Young who was able to control and finish into the roof of the net.
Portsmouth manager Avram Grant:
"I don't like to lose, even a friendly game, but the most important thing for us is the league.
"We played with players who didn't play in the last game. That's good for them because we will need to use them later in the season.
"A few of them did well. Some of the others will have to wait for the next game."
Aston Villa manager Martin O'Neill:
"I was really pleased with him (Downing). What pleased me most was his ability on the ball.
"He can manoeuvre it, he can play and he's got a turn of pace when he's fully wound up.
90:00+4:12 The game is over, as the referee blows his whistle.
90:00+2:16 Foul by Nwankwo Kanu on Luke Young, free kick awarded. Bradley Guzan restarts play with the free kick.
90:00+1:23 The ball is swung over by Hermann Hreidarsson, Richard Dunne gets a block in. Outswinging corner taken by Nadir Belhadj from
the left by-line, clearance made by Carlos Cuellar.
89:14 Substitution (Aston Villa) makes a substitution, with Nathan Delfouneso coming on for Emile Heskey.
88:22 Assist on the goal came from Gabriel Agbonlahor.
88:22 GOAL - Ashley Young:Portsmouth 2 - 4 Aston Villa Ashley Young finds the net with a goal from deep inside the penalty box high into the middle of the goal. Portsmouth 2-4 Aston
88:20 Gabriel Agbonlahor produces a cross,
86:59 Booking Gabriel Agbonlahor is cautioned.
86:55 Frederic Piquionne provided the assist for the goal.
86:55 GOAL - Nwankwo Kanu:Portsmouth 2 - 3 Aston Villa A goal is scored by Nwankwo Kanu from close in low into the middle of the goal. Portsmouth 2-3 Aston Villa.
86:29 Free kick awarded for an unfair challenge on Nadir Belhadj by Gabriel Agbonlahor. Free kick taken by Tal Ben-Haim.
85:50 Free kick awarded for an unfair challenge on Gabriel Agbonlahor by Hermann Hreidarsson. Ashley Young restarts play with the
84:00 Free kick crossed by Nadir Belhadj, save by Bradley Guzan.
84:00 Substitution Marc Wilson is brought on as a substitute for Richard Hughes.
84:00 Stiliyan Petrov challenges Nwankwo Kanu unfairly and gives away a free kick.
83:32 Effort on goal by Ashley Young from just inside the penalty area goes harmlessly over the target.
81:57 Stephen Warnock takes the direct free kick.
81:57 Booking Younes Kaboul is cautioned by the ref for unsporting behaviour.
81:42 Younes Kaboul gives away a free kick for an unfair challenge on Ashley Young.
80:31 Stewart Downing produces a left-footed shot from outside the box and misses left.
80:12 Nadir Belhadj delivers the ball, Frederic Piquionne takes a shot. Save made by Bradley Guzan.
79:53 A cross is delivered by Stewart Downing, save made by Asmir Begovic.
79:24 Short corner worked by James Milner.
78:04 Tal Ben-Haim gives away a free kick for an unfair challenge on Ashley Young. The free kick is swung in right-footed by Stewart
Downing, Younes Kaboul manages to make a clearance. Shot from deep inside the area by Stephen Warnock clears the crossbar.
76:35 The ball is sent over by Ashley Young, save by Asmir Begovic.
75:49 Foul by Tal Ben-Haim on Gabriel Agbonlahor, free kick awarded. Direct free kick taken by Carlos Cuellar.
74:54 Free kick taken by Tal Ben-Haim.
74:54 Substitution Frederic Piquionne replaces John Utaka.
74:54 The official flags Emile Heskey offside.
73:41 The assist for the goal came from Ashley Young.
73:41 GOAL - Stewart Downing:Portsmouth 1 - 3 Aston Villa Stewart Downing finds the back of the net with a headed goal from close range. Portsmouth 1-3 Aston Villa.
73:35 The ball is sent over by Ashley Young,
69:54 Corner from the right by-line taken by Nadir Belhadj, save by Bradley Guzan.
68:41 Free kick awarded for an unfair challenge on Anthony Vanden Borre by Gabriel Agbonlahor. Free kick taken by Michael Brown.
68:10 Shot from 18 yards from Richard Hughes. Bradley Guzan makes a comfortable save.
64:53 Substitution Nwankwo Kanu on for Danny Webber.
Correction - 64:01 Corner taken right-footed by Ashley Young from the left by-line to the near post, Nadir Belhadj makes a clearance.
64:01 Ashley Young takes a inswinging corner to the near post, Nadir Belhadj makes a clearance.
63:54 The ball is crossed by Stephen Warnock, clearance made by Tal Ben-Haim.
61:57 Gabriel Agbonlahor gives away a free kick for an unfair challenge on Tal Ben-Haim. Asmir Begovic takes the direct free kick.
60:42 Free kick awarded for a foul by John Utaka on Carlos Cuellar. Direct free kick taken by Richard Dunne.
58:47 The free kick is delivered left-footed by Nadir Belhadj from right wing, save by Bradley Guzan.
58:47 Booking James Milner receives a yellow card for unsporting behaviour.
58:38 Free kick awarded for a foul by James Milner on Anthony Vanden Borre.
57:43 The ball is sent over by John Utaka.
54:59 Booking Anthony Vanden Borre receives a yellow card for unsporting behaviour.
53:44 Ashley Young takes a shot. Save made by Asmir Begovic.
52:18 Free kick crossed right-footed by James Milner.
52:18 Booking Michael Brown booked.
52:02 Michael Brown challenges Ashley Young unfairly and gives away a free kick.
50:19 Unfair challenge on Younes Kaboul by Stephen Warnock results in a free kick. Asmir Begovic takes the direct free kick.
49:02 The referee penalises John Utaka for handball. Bradley Guzan takes the direct free kick.
47:50 The ball is swung over by Nadir Belhadj, save by Bradley Guzan.
46:21 The ball is swung over by Ashley Young, Asmir Begovic makes a save.
45:21 Danny Webber takes the inswinging corner, clearance by Emile Heskey.
45:01 The referee gets the second half started.
45:00+2:15 The ref blows to end the first period.
45:00+1:33 The ball is crossed by Ashley Young, Header on goal by Gabriel Agbonlahor from inside the box goes harmlessly over the target.
Correction - 45:00+1:28 The ball is swung over by Ashley Young, Header from deep inside the area by Gabriel Agbonlahor goes over the bar.
45:00+0:05 Stewart Downing sends in a cross, Richard Hughes manages to make a clearance.
44:28 Emile Heskey challenges Hermann Hreidarsson unfairly and gives away a free kick. Nadir Belhadj takes the free kick.
40:03 Free kick awarded for a foul by Carlos Cuellar on John Utaka. Anthony Vanden Borre restarts play with the free kick.
38:01 The official flags John Utaka offside. Bradley Guzan takes the free kick.
37:38 Hassan Yebda fouled by Stephen Warnock, the ref awards a free kick. Younes Kaboul takes the free kick.
36:57 Free kick awarded for a foul by Hassan Yebda on Ashley Young. Bradley Guzan takes the free kick.
36:16 The ball is swung over by Hermann Hreidarsson, Bradley Guzan makes a save.
33:58 Shot from just outside the area by Nadir Belhadj goes over the bar.
32:46 Unfair challenge on Nadir Belhadj by James Milner results in a free kick. Nadir Belhadj takes the free kick.
31:56 Michael Brown fouled by Ashley Young, the ref awards a free kick. Younes Kaboul takes the direct free kick.
29:54 Hermann Hreidarsson concedes a free kick for a foul on Gabriel Agbonlahor. Free kick taken by Stiliyan Petrov.
28:41 Hassan Yebda has an effort at goal from outside the area missing to the wide right of the goal.
26:45 Assist on the goal came from Ashley Young.
26:45 GOAL - James Milner:Portsmouth 1 - 2 Aston Villa James Milner finds the back of the net with a goal from just inside the area to the bottom right corner of the goal. Portsmouth
1-2 Aston Villa.
24:47 A cross is delivered by Stephen Warnock, Tal Ben-Haim makes a clearance.
22:32 Gabriel Agbonlahor takes a shot from 25 yards. Save by Asmir Begovic.
21:48 Luke Young takes a shot from just inside the penalty box which clears the bar.
19:15 Centre by Hermann Hreidarsson.
17:51 Inswinging corner taken by Nadir Belhadj from the right by-line played to the near post, Stiliyan Petrov makes a clearance.
Nadir Belhadj decides to take a short corner.
17:21 Handball decision goes against John Utaka. Bradley Guzan restarts play with the free kick.
16:08 The referee blows for offside. Free kick taken by Richard Dunne.
14:42 Free kick awarded for an unfair challenge on Younes Kaboul by Gabriel Agbonlahor. Asmir Begovic takes the free kick.
13:28 Emile Heskey fouled by Tal Ben-Haim, the ref awards a free kick. Direct free kick taken by Stiliyan Petrov.
11:36 Stephen Warnock provided the assist for the goal.
11:36 GOAL - Emile Heskey:Portsmouth 1 - 1 Aston Villa Emile Heskey finds the back of the net with a goal from inside the penalty box low into the middle of the goal. Portsmouth
1-1 Aston Villa.
10:57 Free kick awarded for a foul by Michael Brown on Emile Heskey. Stephen Warnock restarts play with the free kick.
9:47 The assist for the goal came from Danny Webber.
9:47 GOAL - Stiliyan Petrov:Portsmouth 1 - 0 Aston Villa Own goal by Stiliyan Petrov. Portsmouth 1-0 Aston Villa.
9:15 Danny Webber takes a inswinging corner from the left by-line played to the near post.
8:09 Michael Brown fouled by Emile Heskey, the ref awards a free kick. Anthony Vanden Borre takes the direct free kick.
0:00 The ref blows the whistle to begin the match.
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