The Ospreys have won four and lost just one of their five games to date and share top spot with the Dragons who sit out of tonight's fixtures.
The Ospreys defeated Blues North 25-8 in September, but such has been the imporvement of the North, that this game could be a real thrilling contest.
In the other game, the Scarlets, who have lost all five games this season, entertain Blues South, who have a chance of reaching the final having won two of their three games to date.
Blues high performance manager Gafyn Cooper said, "We have had some important games this season but thesetwo matches are crucial to the outcome of this season."
Scarlets v Blues South (Stradey Park) - kick off 7.15pm
Blues North v Ospreys (Abercynon) - kick off 7.15pm
Blues North Team: Owen Morgan, Lewis Williams, Dan Lawrence, Gary Williams, Jordan Jones, Harry Allen, Rhys Downes, Daniel Lewis, Sam Studley, Geriant Evans, James Downes, Gareth Kaplin, Marcus Lewis, Rhys Williams, Ben Lee
Replacements: Liam Kumar, Paul Tampsit, Liam Brabum, Johnny Guy, Matthew Howells, Luke Price, David Miles
Blues South team to play Scarlets:
James Loxton, Nick Gill, Michael Bendon, Will Rees, Joe Xiberras / Solomon Matthews, Joe Deacon, Tom Marley, Daniel Holliday, Carl Fish, Kyle Halley, Owen Sedgebeer, Ryan Moore, Anthony Tennata, Cory Hearne (c), Rhys Meredith
Replacements: Liam O'Brien. Leon Williams, Ollie Morris, Liam Usman, Freddie Wilson, Luke Fear, Lloyd Collier.
In the next round of fixtures Blues South entertain Blues North at Cardiff Arms Park on 8th March, a game not to be missed.