|Pro12 Judgement Day|
|Cardiff Blues (9) 27|
|Tries: Summerhill, Navidi Con: Anscombe Pens: Anscombe 5|
|Ospreys (7) 40|
|Tries: King, Dirksen 2, Underhill, Webb 2 Cons: Biggar 5|
Ospreys fought back from a 10-point deficit to claim their 10th consecutive win over Cardiff Blues in a Judgement Day thriller.
Rhys Webb's two late tries clinched an extraordinary win which ended Blues' European Champions Cup qualification hopes while maintaining his team's.
Ospreys hit back from 17-7 behind as Hanno Dirksen' (2) and Sam Underhill added to James King's first-half try.
Aled Summerhill and Josh Navidi crossed for the Blues.
Ospreys moved into seventh place in the Pro12 table with a home game to come against Ulster on the final day of the regular season, next Saturday.
The Swansea-based team could clinch a place in the Champions Cup next season if other results go their way.
Having led earlier in the game, Blues were 27-26 ahead with six minutes remaining, before Webb's dramatic intervention.
It was a treat for a record crowd of more than 60,000 for this event as the lead changed hands seven times.
In the first instalment of a Judgement Day double-header at the Principality Stadium, Ospreys did not take one penalty attempt at goal as they strove for a bonus point.
Blues just needed to win, and with Gareth Anscombe looking assured at fly-half and the back-row regularly turning over Ospreys possession, they looked well capable of it.
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Anscombe's four penalties and a superb pass for Summerhill's try gave them a 10-point cushion early in the second half.
Wing Dirksen's two tries in five minutes ignited an outstanding finish.
Navidi took advantage of slack tackling before Underhill's try edged Ospreys back ahead.
Anscombe's fifth penalty gave Blues a point advantage before Webb's opportunism took the game away from them in the breathless finale.
If Ospreys beat Ulster with a bonus point in their final game, they could overtake Munster and claim sixth place and with it Champions Cup qualification.
But as well as contemplating defeat, Blues - and Wales - were also left with a fresh injury worry over flanker Sam Warburton.
Cardiff Blues: Dan Fish; Aled Summerhill, Garyn Smith, Rey Lee-Lo, Tom Isaacs; Gareth Anscombe, Lewis Jones; Gethin Jenkins (capt), Matthew Rees, Taufa'ao Filise, Josh Turnbull, James Down, Sam Warburton, Ellis Jenkins, Josh Navidi.
Replacements: Kristian Dacey, Brad Thyer, Dillon Lewis, Jarrad Hoeata, Manoa Vosawai, Tomos Williams, Jarrod Evans, Gavin Evans.
Ospreys: Dan Evans, Hanno Dirksen, Jonathan Spratt, Owen Watkin, Ben John, Dan Biggar, Rhys Webb (captain); Nicky Smith, Scott Baldwin, Dmitri Arhip, Adam Beard, Rynier Bernardo, Olly Cracknell, James King, Dan Lydiate.
Replacements: Sam Parry, Gareth Thomas, Aaron Jarvis, Rory Thornton, Sam Underhill, Brendon Leonard, Sam Davies, Josh Matavesi.
Referee: Nigel Owens (WRU)
Assistant referees: Leighton Hodges, Martyn Lewis (both WRU)
Citing commissioner: Dennis Jones (WRU)
TMO: Jon Mason (WRU)