The Ospreys held their nerve to secure the Judgement Day spoils on an evening of drama at the Principality Stadium, the win ensuring a home tie in the Champions Cup play-off next month.
It was a straight shoot out in Cardiff with a place in the Champions Cup play-offs at stake, the Ospreys needing a draw, their opponents a win, to clinch the knock-out spot.
Sam Davies kicked the Ospreys into an early lead, on exactly four minutes, a straightforward shot at goal after a Blues defender was penalised for not rolling away.
The home team struck back instantly with the first try of the evening, a brilliant solo score from Ospreys bound Gareth Anscombe.
It stemmed from a lineout on halfway, Owen Lane and Lloyd Williams combining to feed Anscombe some 30m out. His jinking run, cutting infield, took him past four defenders to score, before he picked himself up to convert his own score.
Anscombe then stretched the led to seven points, a kick from the touchline after hands in the ruck from Cory Allen.
It was all very low-key with neither side really taking control, but a second successful penalty from Davies brought the Ospreys back to within four points at the start of the second quarter.
A powerful Blues drive at an Ospreys put-in on their own 22 had the visitors in trouble, Olly Robisnon mugging Aled Davies to steal possession as he tried to get it away but Jarrod Evans’ dink to the corner was too long for Jason Harries to get to.
Just a minute or so later the Ospreys were in front. James King, on his 200th appearance, picking up at the back of scrum five, only to be hauled down inches from the line. It was recycled quickly and there was no stopping Allen from close range, under the posts to leave Davies with the easiest of conversions.
With the Ospreys starting to take the game by the scruff of the neck, two trademark Dan Evans runs brought the traveling support to their feet. However, the second time saw play brought back for a Blues penalty, Scott Baldwin easing a defender out of the way in midfield to make space for his fullback.
With the Blues able to go for the corner they initially lost possession due to an overthrow, only to gain a penalty after good work on the ground from skipper Nick Williams. Anscombe duly took his, and his team’s, tally to 13, levelling it with four minutes left of the opening 40.
A neck roll from Rhys Gill on Allen immediately gave the Ospreys a chance to nudge back in front but Davies’ kick from the right never looked like hitting the target.
A minute later Davies had another go, this time on the opposite side of the field, the Blues penalised on their line after Hanno Dirksen closed down Harries, chasing Allen’s grubber. This time the kick was good and the Ospreys led as the teams headed back to the shed.
HALF-TIME: CARDIFF BLUES 13-16 OSPREYS
The Ospreys had their tails up and try number two arrived less than five minutes after the restart.
Dan Evans put the ball into the corner where the Ospreys were able to reclaim possession off the Blues throw five metres out and, eventually, it was Nicky Smith who powered over from close range, Davies with the extras.
Sniffing blood, Allen then went for an interception on halfway that would have given him a clear run to the line. However, the ball went to ground and referee Nigel Owens went to his pocket for a yellow card, ruling it a deliberate knock-on.
The Blues struck immediately, but it was a messy, fortuitous score. They made a hash of the lineout from the kick to the corner but Aled Davies and George North got in each others way, allowing Josh Turnbull to collect and force his way over.
Anscombe converted, the lead was down to just three points and, the Blues thought they had nudged ahead a minute or so later, after a lovely run from Jarrod Evans who fed Tomos Williams the scoring pass.
However, it was referred upstairs, the TMO ruling out the score with one glance at the replay, Nick Williams creating the gap for Evans with a block on Owen Watkin, similar to the penalty against Baldwin in the first half.
With Allen returning to the field and the lead down to three points going into the final quarter the Ospreys were looking to reassert control but the game was being played out mainly between the 22s.
Hands in the ruck from the Ospreys allowed Anscombe to draw his current team level, 23-23, with just 12 minutes to play.
In what looked like a pivotal moment, Dan Evans was shown a yellow card in the 69th minute for what was ruled a deliberate knock-on as he stretched to intercept Rey Lee-Lo’s pass after the centre had latched onto Anscombe’s dink over the top.
In dramatic circumstances, Anscombe’s kick was short, falling to the left of the sticks, leaving it all-square.
A draw would be enough to keep the Ospreys ahead of their opponents to secure the Champions Cup play-off spot, but the Blues, with the extra man, needed the win.
However, the Ospreys were able to strangle the life out of their opponents in the final minutes and they made the game safe with less than 60 seconds remaining on the clock, a penalty for not rolling away allowing Davies to seal the win.