Matters were even for most of the opening half until Phoenix prop Lucy Cahill drove over from a tap penalty. Senghenydd responded from the restart with Hollie Dempsey levelling matters.
Natalie O’Shea then restored the home lead, before a Sophie Slater try converted by Sarah Harper put Senghenydd in a 10-12 lead at the interval.
With the loss of several injured players during the break the second half was always going to be an uphill task for Phoenix, in particular the loss of two of their most pacey players Leanne Burnell and Hannah Seward, whose collar bone injury will probably keep her out for the rest of the season.
Harper got a converted try for the visitors before O’Shea got her second with a 40m effort.
In a seesaw second half Phoenix to their credit fought back from 15-24 down to lead 25-24 , the combative O’Shea storming over for a further two Phoenix tries, and Kaitlin Protheroe getting one for the visitors.
Phoenix took the final play drop out from their own 22 and the visitors countered from deep in their own half before the ball reached Harper on the left hand touchline and she needed no second invitation to run 50m for the winning score before adding the extras.
For Senghenydd Sarah Harper and Jess Kidley were chosen as their players of the match with Natalie O’Shea and Lauren Aitken getting the Phoenix accolade.
Harper stuns Pencoed with late riposte
Despite an equal share of the try count, Pencoed Phoenix went down 25-31 to an agonising last play defeat against Senghenydd.