Ospreys produced a gutsy second-half comeback to edge a nail-biting Welsh derby against Cardiff.

Owen Lane and Mason Grady had seemingly put Cardiff in control with tries at a rain-lashed Brewery Field.

Tinus de Beer kicked his way to the top of the league points tally, but Ospreys' pack wrestled back control marked by Morgan Morse's solo try.

Victory saw Ospreys move up to eighth in the United Rugby Championship (URC) - although two missed late kicks meant Cardiff continued their record of picking up points in every league game.

It would have been more than a losing bonus had they kept their discipline.

The appeal of the festive derbies continued to defy the atrocious conditions with a sell-out 8,159 crowd squeezed inside Brewery Field for this meeting of Wales' top-two sides.

Conditions were so bad that live BBC Wales television coverage was lost then abandoned mid-way through the second half.

This was rugby in the raw, a day for the purists as the passion and competitiveness defied the elements.

Amid the mud-wrestling, individuals emerged to swing the game one way then the other.

Tomos Williams was again superb in controlling the first-half while 18-year-old Morse splashed through pools of water for the try of the game before Wales Under-20s team-mate Dan Edwards steered Ospreys to victory.

Ospreys started strongly as Owen Watkins marked his 100th appearance and saw Giles skip over in the left corner after Jack Walsh and George North combined.

But Cardiff had the better game plan and, in Williams, had its perfect exponent.

The scrum-half's kicking kept turning Ospreys onto the back foot which, in such wet conditions, caused havoc.

His kick and Cardiff's dominance at the breakdown saw them equalise on 16 minutes. Liam Belcher and Lopeti Timani went close before Lane - in for the injured Josh Adams - barged his way over.

Ospreys lost flanker Harry Deaves to an early injury and were soon behind, again from Cardiff's kicks.

Walsh missed a high ball from opposite number Cameron Winnett that landed in the hands of Grady, who had a clear 40m run to the posts.

De Beer converted and added two more penalties as Williams continued to pull the strings. However Cardiff celebrated an impressive goal-line stand, led by lock Seb Davies, just before half-time as though they had scored.

It got worse for the Ospreys before it got better when Deaves' replacement Tristan Davies' head injury meant Dewi Lake was pressed into action as a makeshift back row. But the Wales hooker helped galvanise the comeback.

For the second game in a row, Cardiff conceded a penalty try and a yellow card from a line-out drive, this time James Botham was singled out for the sin bin.

Cardiff again repelled an Ospreys driving line-out only to concede from the restart as Morse gathered on halfway and stepped past three tacklers to score between the posts.

It was a remarkable score from the 18-year-old in such conditions and roused Ospreys for the final push.

Edwards added the conversion and then put Ospreys ahead 22-21 with a penalty before his clever chip switched the direction of attack and allowed Giles to score his second and seal the win.

Ospreys head coach Toby Booth said: "It was an old-school game in an old-school environment and we're absolutely delighted with the win, especially after being behind at half-time.

"We were naive in the first half and invited Cardiff's kicking game but we had some pretty straight talking at the break, Owen Watkins' 100th appearance was an emotional driver and then our set-piece became a massive part of the game.

"In those conditions it was all about territory and keeping the ball in front of the forwards, and our game managers were impressive in doing that in the second half."

Cardiff head coach Matt Sherratt said: "The first half went as well as it could but that was only half a job. The message was to do the same again but the 8-2 penalty count says it all.

"In those conditions, it was about territory, effort and discipline. The effort was there and we picked up another bonus point away to a good side.

"But silly penalties in the second half gave away territory and if you do that, you're going to get punished."

Ospreys: Jack Walsh; Iestyn Hopkins, George North, Owen Watkin, Keelan Giles; Owen Williams, Luke Davies; Gareth Thomas, Sam Parry, Tom Botha, James Fender, Adam Beard (capt), James Ratti, Harri Deaves, Morgan Morse.

Replacements: Dewi Lake, Gary Phillips, Rhys Henry, Lewis Jones, Tristan Davies, Cameron Jones, Dan Edwards. Luke Morgan

Cardiff: Cam Winnett; Owen Lane, Rey Lee-Lo, Willis Halaholo, Mason Grady; Tinus de Beer, Tomos Williams; Corey Domachowski, Liam Belcher (capt), Keiron Assiratti, Seb Davies, Teddy Williams, James Botham, Ellis Jenkins, Lopeti Timani.

Replacements: Efan Daniel, Rhys Carre, Rhys Litterick, Rory Thornton, Josh Turnbull, Mackenzie Martin, Ellis Bevan, Jacob Beetham

Sin Bin: Botham (49), Turnbull (66)

Referee: Adam Jones (WRU)

Assistant referees: Ben Whitehouse & Ben Breakspear (WRU)

TMO: Sean Brickell (WRU)