Pompey come from behind to send Cobblers crashing out of Leasing.com Trophy
Manager Keith Curle made eight changes from Sunday's FA Cup win but you wouldn't have guessed it from his side's first-half performance as they started well and looked more than a match for their Sky Bet League One hosts.
Town deservedly led through Michael Harriman's first goal for the club 11 minutes in, but Pompey belatedly came to the party and were level shortly before the break courtesy of Leon Maloney's header.
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Hide AdAnd the home side made their superior quality - and fitness - count in the second-half when Marcus Harness crashed home a fine volley to send Pompey into round three and inflict a first defeat in nine on the Cobblers, who can now focus on more important matters.
As expected, Curle made wholesale changes for the trip to Fratton Park as Joe Martin, Reece Hall-Johnson and Harriman were all handed rare starts while Ryan Watson captained the team.
Shaun McWilliams was also back after his appendix operation and he was one of six academy players in the squad, with teenagers Ryan Hughes, Jay Williams, Michael Harding and Jacob Ballinger all on the bench.
The mass changes didn't seem to have any impact on the in-form Cobblers, though, and they were quickly into their rhythm and didn't waste any time in taking the game to their Championship-chasing hosts.
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Hide AdHome goalkeeper Alex Bass got enough behind Matty Warburton's low shot with only two minutes played before Scott Wharton headed wide from Scott Pollock's corner.
But the opening goal wasn't long in coming with Cobblers moving into a deserved lead 11 minutes in.
They worked the ball around nicely in midfield before one wing-back teed up the other as Martin's superb left-wing cross was headed past Bass by the onrushing, and unmarked, Harriman.
Harry Smith dragged a shot wide as the visitors continued to knock the ball around confidently, although Pompey showed signs of life midway through the half when Steve Arnold got down quickly to keep out Gareth Evans' low shot.
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Hide AdThe dangerous Ross McCrorie also saw a shot deflected wide but it would be an exaggeration to say Portsmouth were peppering the visiting goal when their rather innocuous leveller arrived.
Coming seven minutes before half-time, it was all a bit too easy for the hosts as Marcus Harness' deflected cross found Maloney at the back post and he headed into the net.
The rest of the first-half petered out without incident but Pompey came out flying in the second and Brett Pitman couldn't quite generate the power to beat Arnold with a header.
Warburton's long-range shot was held by Bass up the other end before Town created a great chance to restore their lead when Jordan Turnbull and Warburton sliced through the Portsmouth midfield with two incisive passes, but substitute Billy Waters lacked conviction with his finish.
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Hide AdPitman was off target with another couple of headers around the hour-mark but the home side were in front moments later and it was a fine finish by Harness, whose volley from 15 yards crashed into the top corner.
Arnold was at his best to thwart Pitman's latest headed effort and then Curle was forced into a change with Martin unable to continue having picked up an injury, replaced by Williams.
Harding was also on for his Cobblers debut but Curle's men never really threatened an equaliser and only another terrific stop from Arnold, denying former Town loanee John Marquis late on, kept the margin of defeat to one goal.
Match facts
Pompey: Bass, Walkes, Downing, Bolton, Cannon (Close 85), McCrorie (Haunstrup 62), Harness, Raggett, Evans, Maloney (Marquis 62), Pitman (c)
Subs not used: Macgilivary, Burgess, Williams, Harrison
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Hide AdCobblers: Arnold, Hall-Johnson, Turnbull, Wharton, Harriman, Martin (J Williams 69), McWilliams (Waters 51), Watson (c), Pollock, Warburton, Smith (Harding 69)
Subs not used: Cornell, Hughes, Kaja, Ballinger
Referee: Will Finnie
Attendance: 2,413
Cobblers fans: 96