Biarritz hit back in the fourth minute through Yann Lesgourgues, but Mieres converted two penalty goals in the 15th and 21st minutes to push the Warriors back in front.
Lesgourgues responded again as he bagged his second try of the night in the 30th minute to bring his side within one point of the Warriors, as the scores stood at 13-12 when the players went in for half time.
After the break, Lesgourgues was soon back into the action as he kicked a penalty goal to give Biarritz a slender 15-13 lead.
Biarritz did not give up and they were soon back on level terms following good work from Benoit Baby to convert a penalty goal, which was then followed up by a try from Thomas leaving the scores tied at 25-25.
The game was left on a knife edge going into the final 10 minutes and it was Biarritz who dealt best with the pressure, as Thomas scored his second try of the night in the 75th minute to help earn his side a slender victory.