Newport Gwent Dragons 20-28 Scarlets
The Scarlets maintained their push for the RaboDirect Pro12 play-offs with a 28-20 victory over Newport Gwent Dragons.
The Llanelli-based region took their opportunity to pile the pressure on the Ospreys above them.
The two regions are fighting for the fourth semi-final spot and the Scarlets, behind their rivals on try count, knew they had no margin for error.
And they made no mistake against a Dragons side that sit one from bottom of the table - although their three-tries-to-two win means they failed to register a much-needed bonus point.
The Dragons were fuelled by a desire to strike back following a 52-19 humbling at the hands of the Ospreys eight days earlier, as well as an awful record against the Scarlets, who they had not beaten in league rugby since 2008.
But the Gwent outfit spent most of the opening exchanges in defence and the pressure finally paid off when Scarlets fly-half Owen Williams knocked over a penalty, with Wales star Toby Faletau also sent to the sin bin after a succession of team offences.
And the 14-man Dragons were facing a 10-point deficit when star winger George North, subject of transfer interest from Northampton, was put over the whitewash by scrum-half Aled Davies.
But for all their dominance, the Scarlets headed into the break on level terms.
First Dragons scrum-half Jonathan Evans spotted a gap at a ruck and raced over from 25 metres for a fine individual score.
Full-back Tom Prydie converted and then knocked over a penalty with the last kick of the half to make it 10-10.
The second half was a more even contest with Dragons causing problems through fly-half Dan Evans, wing Will Harries and full-back Prydie.
But it was the Scarlets that led 16-13 entering the final quarter thanks to a pair of Williams penalties to one by Prydie.
And, suddenly, the west Walians were in a position to chase the four-try bonus point in the closing stages after quickfire tries by international pair Jonathan Davies and North.
Dan Evans, playing his old region, got a try that his all-action display deserved to make it 28-20 with five minutes remaining.
That was how it stayed, the Scarlets getting four points but left to rue failing to cross the whitewash one more time.
Cardiff Blues 16-23 Ospreys
The Ospreys had to battle to overcome a determined Blues outfit for a 23-16 victory which consolidated their position in the play-offs for this season's RaboDirect PRO12 finale.
Having seen the Scarlets surprisingly miss out on a bonus point against the lowly Dragons in the first match of this Millennium Stadium double-header, the Ospreys knew a simple win would be enough to keep them in the top four on points difference.
However, it was not that straightforward as the Blues went ahead with a superbly constructed try.
The ball was spread along the line for skipper Gavin Evans to burst down the left touchline, feed inside to Harry Robinson and watch as the recent Six Nations Player of the Tournament, Leigh Halfpenny, raced in for a touchdown he then converted from wide out.
Halfpenny's Wales colleague Dan Biggar pulled back three points with a close-range penalty, but missed from the halfway line after the Blues' suspect scrum had offended under pressure.
There was a further blow for the Ospreys when they lost international lock Ian Evans to a leg injury, but within a minute they had taken the lead. Flanker James King burst through a gap to speed over on the left, Biggar converting.
Halfpenny slotted another penalty to level the scores, but the Blues were again pinged at the scrum and this time Biggar made no mistake.
And within a minute the visitors - it was officially a Blues home game - scored their second try.
Number eight Jonathan Thomas sent Justin Tipuric surging down the right touchline and, although Halfpenny bravely stopped his gallop, the ball was moved left. Full-back Richard Fussell and centre Ashley Beck freed Jonathan Spratt, who was able to touch down despite a desperate last-ditch attempted tackle by Rhys Patchell. Biggar added the extras.
The Ospreys opened the second half with a period of steady pressure, but the Blues' defence survived and they moved downfield for Halfpenny to cut the deficit with a well-struck penalty.
He added another from almost the same spot, before Biggar, inexplicably, missed an easy kick at the other end.
A Biggar cross-kick was acrobatically won by Lions captaincy candidate Alun Wyn Jones, who fed Spratt, but the Ospreys knocked on and the opportunity was lost.
The Blues conceded a penalty at the scrum, but the Ospreys opted for a reset rather than take the easy kick, a decision proved wrong when Biggar, having a mixed game, failed to gather a pass and possession was lost.
When the situation was repeated, the Ospreys played it safe and Biggar duly claimed the three points. It proved enough to secure the spoils.
Leinster 18-22 Ulster
Ulster got their RaboDirect PRO12 challenge back on the rails with a hard-earned 22-18 derby victory over Leinster at the RDS.
Tries from Robbie Diack and Iain Henderson helped the Ulstermen record their first away win over Leinster since 1999, giving them a timely boost ahead of next week's Heineken Cup quarter-final against Saracens.
But the visitors had to show a huge amount of defensive steel at the end. They were put through a series of scrums close to their line in a frantic finish, during which they lost Jared Payne to the sin-bin.
Leinster went close through David Kearney and Sean Cronin, the latter being held up by Kyle McCall in the final seconds, as they lost for the first time in the league since Ulster beat them at Ravenhill in December.
Mark Anscombe's side, winless in their last three matches, exerted plenty of pressure in the first half but Ian Madigan was unerring from the kicking tee as Leinster led 12-8 at the break.
Ruan Pienaar was just as impressive from place-kicks, finishing with 12 points and Henderson's 62nd-minute try gave Ulster a lead they failed to relinquish.
Eager Ulster were first to threaten with Paddy Jackson using quick lineout ball to unleash Andrew Trimble through the middle with Darren Cave providing support.
Leinster were quick to shut down the space in their 22, although their provincial rivals continued to display good ball retention in advanced positions.
The hosts absorbed the early pressure and took the lead through the boot of Madigan on the quarter hour, following a good carry from Brendan Macken.
A wind-backed Madigan made it 6-0 four minutes later from further out, converting a long range penalty after Nick Williams was singled out for a ruck infringement.
Ulster sprung Williams forward in the build-up to the game's opening try, the New Zealander charging onto Dan Tuohy's inviting pass and Jackson then flung it wide for Diack to score in the left corner.
Pienaar was unable to convert and Madigan replied for Leinster in the 26th minute, splitting the posts from outside the 22 after Ulster were pinged at a scrum.
As the pace quickened, good hands from Tuohy seemed to release Craig Gilroy for a jinking run to the line - but the lock's offload was correctly ruled forward.
It was another glimpse of Ulster's quality in attack, and their patient approach-work and relentless recycling should have led to points.
Leinster's well-organised defence held them in check until replacement Andrew Conway was caught offside from a kick ahead and Pienaar nailed the resulting penalty.
Ulster's discipline let them down at times and Williams was whistled up for not rolling away quickly enough in injury-time, leading to Madigan's fourth successful penalty of the night.
Increased pressure on the Ulster scrum saw Leinster resume on the front foot with Madigan adding another three-pointer to their total.
Pienaar, with the breeze now behind him, cancelled that kick out with a crisp penalty strike from distance and gave Ulster further momentum with a sure-footed 57th minute kick from the right.
After a defence-dominated third quarter, Ulster began to make headway when putting width on the ball again with Gilroy and Stuart Olding both threatening.
Trimble was brought down just short of the whitewash in the 62nd minute and a quick recycle allowed Pienaar to put replacement Henderson over in the right corner.
The South African failed to add the extras and with their scrum giving way, Madigan split the posts to cut the gap to a single point.
Conway conceded a needless penalty when he tackled Jackson in the air as he gathered a kick, and Pienaar stepped up to make it 22-18.
Time was not on Leinster's side and having turned down a late kickable penalty, the hosts put their faith in their solid set-piece.
It was not enough in the end as despite giving away penalties within scoring range, Ulster - with Rory Best leading by example - defiantly held on for a precious win in Dublin.
Treviso P-P Connacht
The match was postponed at short notice because of a waterlogged pitch.
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- Newport Gwent Dragons