suffering a last-gasp defeat at Zebre - a result that saw the Italian outfit move off the bottom.
The Welsh region went into Thursday night's game chasing Ulster in fourth place but a dramatic late try from Zebre replacement Guglielmo Palazzani dented their cause.
Tries for Alun Wyn Jones, Ashley Beck and Justin Tipuric were not enough as Zebre won 30-27 to leapfrog compatriots Benetton Treviso into 11th.
Ospreys' losing bonus point moved them within one point of Ulster but the Irish outfit cemented their top-four spot despite defeat to Leinster the following day.
Fly-half Paddy Jackson scored all of Ulster's points but Shane Jennings' second-half try was the difference as league leaders Leinster won 22-20 at Ravenhill.
For Ulster the losing bonus point was enough while reigning RaboDirect PRO12 champions Leinster stayed top with the win.
Glasgow Warriors and Munster are still nip-and-tuck for a home semi-final after winning away at Treviso and Edinburgh respectively.
The Warriors hold a one-point advantage after Tommy Seymour's hat-trick led them to maximum points in Italy, winning 38-16.
But Rob Penney's Munster are breathing down their necks, romping to a 55-12 win at Meggetland, courtesy of tries from five different players.
Newport Gwent Dragons left an all-Welsh clash at Parc y Scrlets empty-handed despite a pair of tries for Tom Prydie.
Ken Owens, Gareth Davies, Jordan Williams and Jake Ball each dotted down for Scarlets as they claimed maximum points, ensuring a top-six finish.
And Cardiff Blues edged a close contest at Connacht 22-15, fighting back after a slow start to make it four RaboDirect PRO12 wins in a row and move into seventh.
TRY OF THE WEEKEND
He finished Glasgow's win in Italy with three tries and Tommy Seymour saved the best until last. After receiving the ball directly from a neat lineout move, the wing powered up the touchline unchallenged. He then produced a perfectly-weighted chip-and-chase to beat the first defender, before showing raw speed and power to beat the second and slide over.
BREAK OF THE WEEKEND
Zebre produced late drama when they welcomed Ospreys on Thursday but they had also made a bright start to the clash, led by fly-half Luciano Orquera. As well as kicking them into an early lead, Orquera conjured up a nifty piece of footwork, breaking through the Welsh defence before sending a looping pass out to Giovanbattista Venditti to score the first try.
PLAY OF THE WEEKEND
Munster's win at Edinburgh was often an exhibition of sharp offloading and Andrew Conway's try was perhaps the best example. Conor Murray sent the ball wide from well inside the Munster half and Edinburgh simply couldn't cope with their quick hands, CJ Stander eventually sending Conway away towards the posts.
PLAYER OF THE WEEKEND
Glasgow Warriors are flying high and Tommy Seymour's display in Italy was indicative of a team in confident spirits. The 25-year-old scored three times - including that superb hat-trick try - as the Warriors cruised to their seventh straight RaboDirect PRO12 win and within one game of securing a home semi-final.
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