Cardiff Blues Under-18s were edged out 13-8 by Ospreys Under-18s in the Regional Championship clash at BT Sport Cardiff Arms Park - to leave the race for the final still wide open.
Defending champions Blues went into the game knowing a win would book a place in the final. But a determined display from Ospreys avenged defeat when the sides met in November.
The Blues were also hit by an injury to scrum half Carl Lewis who was stretchered off in the second half.
Blues are next in action a week on Wednesday when they face Scarlets eager to book a final spot.
A superb early break from Owen Lane caused alarm for Ospreys as he broke down the middle as the home side dominated the opening quarter without breaking through.
The visitors showed they were a real threat as winger Keelan Giles scampered clear after neat handling by the touchline, but was called back for a forward pass in the build-up.
Blues back row Morgan Sienawski stole a lineout to charge forward, yet it was Ospreys that could have taken the lead when Matthew Aubrey missed a penalty wide right.
Harri Lang’s tap tackle stopped a certain try but Ospreys were not to be denied the lead as Aubrey this time found his range.
The rain got heavier as the half wore on the forwards used it to their benefit as they kept the ball tight with a big driving maul before spinning it infield for Teri Morgan Gee to crash over for a 5-3 lead at the break.
A penalty for offside saw home fly half Callum Hall push a kick wide before hooker Jordan Liney was sin binned for dragging down a driving maul.
The next scrum saw Blues, with an extra man, win the penalty and Hall kicked to the corner to try and drive home the advantage. Corey Domachoski was then held up as he drove for the whitewash.
Ospreys struck as a well worked try saw Phil Jones over in the right corner before a lengthy delay to the game as scrum half Carl Lewis was stretchered off with a leg injury.
And the visitors went further ahead with a score from James Davies as he powered over from close range.
Hall cut the lead back to 13-8 with a well struck penalty. However, despite late pressure, there was no late reprieve as Ospreys held on to blow the race for top two wide open.
29m Aubrey p 0-3
32m Morgan Gee t 5-3
50m Jones t 5-8
54m Davies t 5-13
70m Hall p 8-13
Cardiff Blues Under-18s: 15 Harri Lang, 14 Kieron Gay, 13 Owen Lane, 12 Teri Morgan Gee, 11 Curtis Povey, 10 Callum Hall, 9 Carl Lewis; 1 Corey Domachowski, 2 Gerwyn Coleman, 3 Keiron Assiratti, 4 Callum Bradbury, 5 Callum Haggett, 6 Aled Ward, 7 Morgan Sieniawski, 8 Joe Gaughan
Replacements: 16 Corey Tarrant, 17 Rhys Carre, 18 Harri Dobbs, 19 Owen Jones, 20 Shane Lewis-Hughes, 21 Tom Lucas, 22 Jesse Lipetz Robic, 23 Cameron Gibbon