Corey McMullan scored a late winner as Ballyclare Comrades beat Institute 1-0 in the first leg of their Championship play-off at Dixon Park on Tuesday.
McMullan drilled a low 20-yard shot into the bottom corner in the 81st minute to ensure his side going into Friday's second leg at Drumahoe ahead.
The winner of the tie will play Carrick Rangers over two legs next week, with the victor sealing a Premiership place.
Warrenpoint Town have already been promoted as Championship winners.
This is the first year that a play-off between the second and third placed teams in the Championship has taken place, Institute having finished second, six points ahead of Comrades.
Ballyclare had the better of the first half and their best scoring opportunity fell to Dean Youle, his effort being kept out by Martin Gallagher.
For the visitors, Mark Scoltock headed over from eight yards from a Michael McCrudden corner.
In the second-half, Gallagher made another important intervention, tipping Chris Trussell's 25-yard free-kick over the bar.
Institute's Stephen O'Donnell saw his header come off the top of the bar, while Sammy Morrow had his shot denied by Paddy Flood.
The team that emerges on top after Friday's second leg will host Carrick, who finished 11th in the top flight, in the first leg of the promotion/relegation play-off on Tuesday 9 May, with Rangers enjoying home advantage for the second leg on Friday 12 May.