However, their one defeat, against the Dragons in October, has left the race for the final wide open with four of the five regional sides having already registered at least one win.
In the other game, the Scarlets take on the Dragons at Llandovery, with the visitors looking to maintain the pressure on the Ospreys with a third win in four games.
The pick of their results came at Newbridge when they turned over the Ospreys 13-12. Another win tonight will certainly put them in with a decent chance of reaching a first final.
Ron Schottle, high performance manager for the Dragons said, "We have been delighted with the progress that the regional sides have made this season, but we have to maintain our high standards.
"This is going to be a particularly tough game against a determined Scarlets side who we beat on the opening game of the season."
Scarlets v Dragons (Llandovery, 7.15)
Ospreys v Blues South (Corus, 7.15)