Mansfield took a two-goal lead with just a little more than half-an-hour played and threatened to kill the tie off there and then, but Town fought back well in the second 45 minutes and will fancy their chances back at Sixfields in Wednesday’s second-leg.
There was not much between the teams in general play before half-time but Mansfield undoubtedly carried the greater threat and that was reflected by their 2-0 half-time lead, given to them by goals from Rhys Oates and Jordan Bowery.
Cobblers improved though and they made all of the running in the second-half, getting themselves right back in the tie through Koiki’s deflected effort before missing chances to equalise.
The teams will do it all again at Sixfields on Wednesday evening to decide who goes to Wembley.
Jon Brady made two changes from the last game of the regular season with Jonny Maxted and Idris Kanu both handed starts.
Regular number one goalkeeper Liam Roberts served his one-game suspension and Josh Eppiah was only fit enough for the bench.
A crackling atmosphere set the scene for a huge game of football and Mansfield were always likely to make a fast start, although Cobblers weathered the opening 10 minutes without too many alarms.
The first shot of the contest came at the other end where the ball dropped kindly to Louis Appéré on the edge of the box and he struck a sweet effort that had to be pushed behind by Nathan Bishop.
But it was Mansfield who made the breakthrough with a controversial opener after 14 minutes.
Jack Sowerby’s pass was intercepted in midfield, the hosts sprung forward and Bowery slipped in an offside-looking Oates who slammed a shot between the legs of Maxted.
The first-half was generally quite even and Cobblers strung together some decent passages of play without creating a clear-cut chance.
However, the visitors also were uncharacteristically open and loose at the back and that allowed a dangerous Mansfield attack to create chances and eventually double their lead.
The second goal arrived on 33 minutes and it was too easy for Jamie Murphy to get free down the left and send in a low cross that was tapped in at the back post by an unmarked Bowery.
That took the wind out of Town’s sails and whilst they did well to get to half-time only two goals behind, it almost became three moments after the restart when the impressive Oates was denied by Maxted.
Sam Hoskins curled a shot a yard wide from the edge of the box but Town’s attack remained disjointed and struggled to properly trouble Mansfield, and Brady duly sent for Eppiah with still over half-an-hour to play.
And Cobblers had one back just five minutes later.
The move began just outside their own penalty box as Shaun McWilliams won back possession and instigated a counter-attack that was carried on by Mitch Pinnock and Appere.
The ball eventually made its way to Koiki and his right-foot shot from just inside the penalty box deflected into the bottom corner.
The tie had turned and suddenly Cobblers were now in the ascendency and pushing for an equaliser.
Hoskins fired in an effort that was well blocked and Eppiah’s cross-cum-shot was nearly turned in by a defender.
A vital defensive intervention from John-Joe O’Toole also prevented a tap-in for Eppiah and Joseph Mills headed a good chance wide but although an equaliser would not come, Town are very much in the tie going back to Sixfields.
Mansfield: Bishop, Perch ©, O’Toole, Hawkins, McLaughlin, Maris, Longstaff (Wallace 60), Quinn (Stirk 77), Murphy (Akins 86), Oates, Bowery
Subs not used: Stech, Hewitt, Clarke, Lapslie
Cobblers: Maxted, Mills ©, Horsfall, Guthrie, Koiki, McWilliams, Sowerby, Pinnock, Hoskins, Kanu (Eppiah 56), Appèrè (Rose 66)
Subs not used: Woods, Harriman, Dyche, Pollock, Zimba
Referee: Anthony Backhouse
Cobblers fans: 1,221