Neath-Swansea Ospreys bade farewell to St Helen's in style by booking a Celtic Cup semi-final at Ulster's expense.
CELTIC CUP QUARTER-FINAL
Ospreys: (10) 23:
Tries: Terblanche, Jones.
Cons: M Jones 2.
Pens: M Jones 3.
Ulster: (3) 16:
Tries: Penalty try.
Pens: Humphreys 2.
The Ospreys will move into their new stadium at Morfa next season, although Swansea RFC will stay at St Helen's.
Young fly-half Matthew Jones led the way with a try, two conversions and three penalties to book an all-Welsh thriller against the Llanelli Scarlets.
Stefan Terblanche scored Ospreys' other try, while Ulster had a penalty try and eight points from David Humphreys.
Terblanche broke the deadlock after 18 minutes when the former Springbok wing spotted space in behind the Ulster defence and chipped through.
Ireland wing Tyrone Howe did well to turn and match Terblanche step-for-step, but it was the Osprey who got the vital touch over the line.
Matthew Jones cooly stroked a difficult conversion from wide on the right and then added a penalty to delight the home support.
Humphreys replied with a penalty for the visitors, but Ulster were finding it difficult to carve out any clear-cut scoring opportunities.
The closest the Irish came was a clever chip to the left in-goal area that was just too high for Paul Steinmetz to gather and score.
Humphreys was showing all his experience kicking from hand and was winning the territorial battle with opposite fly-half Jones.
But the Ospreys showed real threat when attacking from deep, as Wales wing Shane Williams showed after gathering a long kick.
The newly selected Lions tourist strung together a mesmerising series of side-steps that took him past half-a-dozen would-be Ulster tacklers.
Recognising the continuing danger posed by the little jinker, Rod Moore tried to stop Williams taking a quick tap soon afterwards.
But all the former Wallaby prop got for his troubles was 10 minutes in the sin bin.
The Ospreys should have stretched their lead 10 minutes after half-time when centre Elvis Seveali'i broke clear.
Ulster full-back Bryn Cunningham did brilliantly to haul the Samoan down just short of the line, but the ball was recycled quickly right.
However, Jones sent a grubber through when the pass was a better option and the ball beat both defenders and attackers to the dead-ball line.
The young fly-half made amends with a penalty to put Ospreys two scores clear, only for Humphreys to bring Ulster back to 13-9 with two quickfire kicks.
But Jones added his third penalty of the evening, before rounding off the move of the match for a try.
Seveali'i started things with an audacious reverse pass, Jason Spice took it on and then off-loaded for wing Williams to race clear.
As the Ulster tacklers streamed back, Terblanche hit a brilliant angle to take Williams' pass and then shipped the ball on to Jones to score.
Ulster claimed a late penalty try and conversion in the final moments, after Humphries had been hauled down just short.
Ospreys lock Brent Cockbain earned a yellow card for stealing the ball on the floor, but referee Andrew Ireland's next act was to blow for full time.
Although the Llanelli Scarlets await at Stradey Park in an all-Welsh semi-final, the Ospreys remain on course to add the Celtic Cup to their Celtic League title.
Ospreys: A Durston; S Terblanche, A Bishop, E Seveali'i, S Williams; M Jones, J Spice; P James, B Williams (capt), A Millward, B Cockbain, J Thomas, J Bater, R Pugh, R Jones.
Replacements: D Jones, L Bateman, S Tandy, P Connor, D Bishop, R Mustoe, N Bonner-Evans.
Ulster: B Cunningham; T Bowe, K Maggs, P Steinmetz, T Howe; D Humphreys, K Campbell; S Best, P Shields, R Moore, G Longwell, M McCullough, A Ward (capt), G Pfister, R Wilson.
Replacements: R Best, R McCormack, R Frost, C Feather, N Doak, P Wallace, A Maxwell.
Referee: Andrew Ireland (Scotland)