Bridgend Athletic established an early 10-3 lead with a try by flanker Evans and a conversion and penalty from scrum half Davies.
Glamorgan Wanderers kept in the tie with two penalties from outside half Bird before Davies extended the Bridgend Athletic tally with another penalty.
A try in the last moments of the first half from wing Mortimer pulled the Wanderers back into the tie.
At half time Bridgend Athletic led narrowly by 13-11 points.
In the second half Bridgend Athletic extended their lead with another Davies penalty. Glamorgan Wanderers upped their game and through their big and well organised pack exerted pressure on the Bridgend Athletic line.
After withstanding that pressure for some time Bridgend Athletic conceded a penalty try after a driving maul from the now dominant Wanderers pack. With about 10 minutes to go Wanderers took the lead for the first time in the match. They held on to win a hard fough clash.
At Abercynon it was a less tense battle as Merthyr gained a 18-0 victory.
Merthry converted early pressure into points that led to them eventually running out comfortable winners. Merthyr scored two tries, a conversion and a penalty with no reply from Beddau Youth.
It is planned to play the Final on Saturday 1st May at a venue to be determined.