Sean Long and Jake Doyle-Hayes' Cheltenham took a big step towards the League Two play-off final with a 2-0 victory over Alan McCormack and Michael Harriman's Northampton in the semi-final first leg at Sixfields.
The visitors edged the first half and led at the break thanks to Charlie Raglan's header from former Ireland U21 Doyle-Hayes' corner before Conor Thomas struck late on to leave the Cobblers with a mountain to climb.
The home side had enough chances to at least draw but they were repeatedly thwarted by Cheltenham's outstanding goalkeeper Owen Evans, whose early penalty save from Ryan Watson when Harriman was tripped could well prove to be the tie's defining moment.
The two sides will face each other again in Monday's return leg at Whaddon Road with the winners taking on either Colchester or Exeter at Wembley on 29 June.
Cohen Bramall's late free-kick gave Colchester a narrow 1-0 win in the first leg of the other semi-final.
Bramall curled in a set-piece with nine minutes remaining which Exeter goalkeeper Lewis Ward could only parry into the net at his near post, giving the U's the edge ahead of the second leg at St James Park.
Ex Bray Wanderer Pierce Sweeney was at right back for Exeter.
