Cardiff Blues A produced a spirited display on home soil before losing out 24-15 to Ospreys A in an entertaining encounter at BT Sport Cardiff Arms Park.
Blues enjoyed plenty of possession in the first half and were rewarded when they opened the scoring through the boot of Will Thomas.
But a try from visiting wing Tom Grabham two minutes before the break gave the Ospreys a slender 5-3 lead at the break.
Hanno Dirksen grabbed another early in the second half before Blues responded as James Sheekey was driven over from a driving lineout.
However, a penalty try on the hour, after a series of big scrums from Ospreys, saw them pull clear and more power up front set up Callum Davies for a fourth.
Replacement Will Baird grabbed a second home try but the Blues finished just short as the crowd enjoyed an entertaining one-off clash between the two regions.
The home side started the game with plenty of verve and invention with a neat chip from full back Tom Williams almost sending Blues away as he collected his kick at full pace and flicked inside to lock Miles Normandale.
Centre Garyn Smith was next to break the line, as he stepped his opposite number and fed young hooker Ethan Lewis on his shoulder. Lou Reed then rattled into contact but Blues could not finish off of another promising move.
Huge physicality from the hosts was evident in the early exchanges with Tom Pascoe unloading a big tackle to force Ospreys to spill the ball. The alert Harry Davies hacked ahead before a penalty for offside gave Thomas a simple opening kick of the night.
Neat handling from half back pairing Michael Hale and Thomas freed Jevon Groves down the middle. Ethan Lewis continued to be a willing ball carrier and another penalty for not releasing saw Thomas push a kick wide of the target.
Opposite number Luke Price also sliced a kick across the posts, after Blues were penalised for illegally playing the ball on the floor.
A superb piece of handling and offload from Ethan Lewis created space wide and the ball shot through the hands and Smith took it to the line. Davies and Pascoe then went for the whitewash but the ball was spilled forward.
Ospreys came alive just before the break with big Number Eight Ifereimi Boladau bursting down the middle and popping a pass to flanker Lloyd Evans. The visitors kicked to the corner and after the initial drive the ball was flung right for Grabham to race over for a half-time lead.
Solid defence from Blues at the start of the second half held Ospreys at bay before the visitors finally broke through as the forwards burrowed down the middle then quick ball fed Aled Jenkins who was alert enough to offload in the tackle for Dirksen to dive over.
The response from Blues was quick and emphatic as they turned down a kick at the posts, opted for the corner and duly powerfully drove over with Sheekey the man to slam the ball down.
The game was now flowing from end to end with Ospreys back on the attack as Callum Davies nearly broke through. A series of big visiting scrums paid dividends as the referee awarded a penalty try.
Owen Watkin showed neat footwork to set Ospreys on the attack again and more pack power shunted to the line before replacement Davies touched down.
Baird tore over unopposed after spotting a gap at scrum time to set up a big finale, but it was Ospreys who enjoyed territory late on to see out the victory.
09m Thomas p 3-0
37m Grabham t 3-5
48m Dirksen t 3-10
53m Sheekey t 8-10
60m Penalty t Price c 8-17
69m Davies t Price c 8-24
74m Baird t Whittingham c 15-24
Cardiff Blues A: 15 Tom Williams, 14 Harry Davies, 13 Garyn Smith, 12 Tom Pascoe (Phillips 61), 11 Elliott Kear (Wyn Benham 56 (Coleman 74)), 10 Will Thomas (Whittingham 61), 9 Mike Hale (Baird 51); 1 Thomas Davies (C Lewis 40), 2 Ethan Lewis, 3 Dillon Lewis (Jones 68), 4 Miles Normandale, 5 Lou Reed (Davies 27-36, 40), 6 Jevon Groves, 7 Rory Watts-Jones (Sheekey 37), 8 Ben Roach
Replacements: 16 Gerwyn Coleman, 17 Callum Lewis, 18 Joe Jones, 19 Seb Davies, 20 James Sheekey, 21 Will Baird, 22 James Whittingham, 23 Ellis Wyn Benham, 24 Jack Phillips