Culter huffed and puffed and eventually overcame a spirited Thistle side with a late barrage of goals but it could have been so different as Forres missed a penalty when they were 1-0 up.
Culter now meet Banks O’ Dee in the Semi-final of the North Regional Cup at Spain Park.
The home side started well with Ryan Stott and Adam Wilson both going close before Ross McWilliam almost opened the scoring but Simon Bremner pulled off a great save.
With only 15 minutes played a cute Ryan Stott free kick was once again saved by the Forres ‘keeper.
The Deesiders were stunned in the 26th minute when a pot shot by Jordan Robertson flew past a ruck of defenders and nestled in the back of the net to give Forres a surprise 1-0 lead.
A few minutes later Adam Wilson brought down Matt Davidson in the box but the same Forres player crashed the resulting penalty off the underside of the bar.
The visiting ‘keeper then pulled off a fantastic save from a Greg Donald strike. Thistle were dealt a severe blow with 40 minutes gone when Matty Fraser was shown a red card for a second hand ball offence.
Culter missed a host of chances at the beginning of the second half and eventually sent on Lewis McKimmie and Allan Youngson for Lee Murray and Richard Wyness, but they still failed to take a series of good chances.
With the clock running down, Ross McWilliam was downed in the box and Ryan Stott stepped up to equalise in the 81st minute.
A few minutes later Culter got their noses in front for the first time when Allan Youngson picked out Ryan Stott and the winger crossed for Ross McWilliam to hammer home from close range after 85 minutes.
Deep into stoppage time Lewis McKimmie scored a brilliant third when he darted though a posse of defenders and slipped a great finish under the Forres ‘keeper to send Culter to the semi-final.