Dan Biggar kicked six penalties to steer the Ospreys back from the brink of defeat as they overturned a 13 point deficit to beat Ulster 23-22 at the Liberty Stadium.
The axed Welsh outside half guided the Magners League champions to a much needed victory that keeps them in touch with the top four teams on a weekend when two of them slipped up away from home.
The Scarlets were pipped in Connacht and runaway leaders Munster were overturned in Treviso, so the Ospreys needed to make the most of the weekend by closing the gap on the front runners for the four Play-Offs places on the road to the second Magners League Grand Final on Saturday, 28 May.
The four points looked a long way off after Ian Humphreys had given Ulster a 13-0 lead with two penalties and then a penalty try which he also converted. In much the same way as they had blown away Cardiff Blues on their last visit to Wales, when they ran out 37-15 winners, Ulster scored at almost a point a minute in the opening quarter to create what looked like being a winning platform.
But after that bright opening the Ospreys scrum began to turn the screw and the penalties flowed as the Ospreys finally got their act together.
That allowed Biggar to prove he can still hit the mark with his goalkicking as he landed six out of eight attempts to punish the visitors. One of his penalties was a monster from 55 metres on the stroke of half-time that gave the home side the lead for the first time at 14-13.
As well as Biggar's three penalties the Ospreys also conjured up a corner try for ex-All Black flanker Marty Holah which owed much to the skill and vision of Ashley Beck, who threw a long, cut out pass to create the opening.
Two silly home indiscretions at the start of the second half allowed Pienaar, who took over at outside half from Humphreys when he left the field with a shoulder problem after 31 minutes, to show off his considerable goalkicking skills and Ulster suddenly found themselves five points clear.
But Biggar replied with three more penalties, two for scrum offences, and not even a third strike from Pienaar six minutes from time could claw back the deficit.
Scorers:Ospreys: Try: M Holah; Pens: D Biggar 6. Ulster: Penalty Try; Con: I Humphreys; Pens: R Pienaar 3, I Humphreys 2
Angus O'Brien has quickly settled into the Wales Sevens squad after solid performances in Hong Kong and Japan. With two rounds remaining in Glasgow and Twickenham on the World Series, WRU TV's Graeme Gillespie chats to the rising star.
Wales winger Luke Morgan has established himself as one of the deadliest finishers on the Sevens World Series. WRU TV's Graeme Gillespie catches up with him on the eve of this weekend's round in Glasgow.
All roads lead to Cardiff on Sunday for the SSE SWALEC Finals Day at the Millennium Stadium. Cambrian Welfare tackle Ystradgynlais in the Bowl final, Ystrad Rhondda play Newcastle Emlyn in the Plate final while defending champions Pontypridd face Bridgend in the Cup final.
WRU Chief Executive Roger Lewis, Deputy Minister for Culture, Sport and Tourism Ken Skates and WRU Chairman Gareth Davies earlier this week launched a new competition in conjunction with the WRU unveiling its archive room.
Welsh players and management joined Deputy Minister for Sport Ken Skates at the Pierhead Building, Cardiff Bay earlier this week to reveal the Union's pioneering school-club hub initiative has been so successful the project will now almost double the scale of the original project.
The Scarlets, Newport Gwent Dragons, Cardiff Blues and the Ospreys look ahead to Judgement Day III which sees the Blues take on the Ospreys (2.30) and the Dragons tackle the Scarlets (4.45pm) at the Millennium Stadium tomorrow afternoon.
Ystradgynlais and Ynysddu played out a titanic struggle at Taffs Well in the SSE SWALEC Bowl semi-final. At the end of the game neither team could be separated after a 15-15 stalemate but Ystradgynlais go through on try count.
Blues strike first in JD III battle - Newport Gwent Dragons lock Andrew Coombs, WRU referee Dylan Llyr Jones and Cardiff Blues hooker Matthew Rees decide dressing room allocations for Judgement Day III
Wales Under 18 captain Calum Haggett has received a conditional offer to read Biomedical Science at St Anne's College, Oxford. WRU chairman Gareth Davies went to meet Calum and Coleg y Cymoedd Principal Judith Evans to wish him luck in his exams and pass on some of his personal experience.