There is, as they say, a first time for everything and they will be saying that all week in Ulster and South Wales. Ulster return to Glasgow for the first Play-Off on Friday night, the Ospreys to Thomond Park for the second Play-Off on Saturday afternoon.
All four contenders know what it takes to get to the Guinness PRO12 Final although only one of the quartet, the Ospreys, have made it there more than once. While Ulster (2013) and the Warriors (2014) have lost Dublin finals, the Welsh challengers have won there twice, at Leinster's expense in 2010 and at their further expense two years later.
All four need no lessons on the importance of home advantage. The Warriors have lost three away semi-finals, at Leinster twice and Ospreys once. Munster have also lost three on the road, in Glasgow last season, Swansea (2012) and at the RDS Arena in 2010.
Both home semi-finalists had to endure an afternoon of high tension and higher anxiety before securing their due reward for finishing in the top two. They got there in the end but only after a thrilling climax to the regular season which could just as easily have forced one or both to take to the skies for their play-offs.
Warriors, 6-10 down to a depleted Ulster at half-time after opting to play into the wind and rain at Scotstoun, knew that only a bonus-point win would prevent them from being blown off course. With the best part of an hour gone, they hadn't scored one try, let alone four.
Stuart Hogg's stylish chip-and-chase opener paved the way for two more from Finn Russell followed by the clincher from Richie Vernon. Four tries in barely 20 minutes ensured the Warriors finished in the top two, deservedly so as the only team to return a perfect home record - eleven wins out of eleven.
Ulster, whose only win on Clydeside in the last four seasons came in the European Cup, go back there on Friday with a very different team to the one beaten six days earlier. The best they have got will be straining at the leash for 80 minutes that offers them the challenge of saving their season from subsiding into an anti-climactic finish.
Another sell-out near-10,000 will pack Scotstoun to the rafters for the second week running, the vast majority ready to burst their lungs in support of a home win which would leave Ulster in the ironic position of being on the outside looking in on Belfast's first GUINNESS PRO12 Final.
The Warriors will not settle for the worthy recognition bestowed upon them during the Guinness PRO12's glittering awards dinner at the Guinness Storehouse in Dublin where Gregor Townsend won Coach of the Season and skipper Ali Kellock received the Chairman Gerald Davies' award for his outstanding services to the game in Scotland.
The big prize which eluded them during last year's Grand Final against Leinster at the RDS, is still to be won. Townsend's take on the semi-final has set the mood: "We'll have to play a lot better than we did last Saturday.''
Kellock's largely all-Scottish squad will not be caught resting on any laurels this week, not least because they will argue that they don't have any to rest on. Ulster will be driven by the grim reality that nine years have come and gone since they won their only PRO12 title.
Munster, all five points in the bag well before half-time against the Dragons in Cork, owed their top-two finish in the final analysis to some neighbourly assistance from Connacht in Galway. Having conceded 24 points at the rate of almost one a minute, they recovered to run the Ospreys close and, crucially, deny them the one more try to add to their early trio.
The extra point would have put Steve Tandy's in-form contenders in second place and squeezed Munster into third by virtue of one more win over the season. In that event, Saturday's Play-Off would have been on the Welsh side of the Irish Sea.
Munster have lost one of their last ten matches, to the Ospreys during the RBS 6 Nations. Coach Anthony 'Axel' Foley needs nobody to remind him that the Welsh birds of prey have done the double over his native province this season.
''Some team is going to lose a home semi,'' he said in the aftermath of the 50-27 win over the Dragons. ''It's important that a team that's beaten us twice this year doesn't make it three. It's going to take a lot of hard work and we'd appreciate the help of Thomond Park.''
He will get it, not that that will bother the Ospreys and the newly-elected Guinness PRO12 Players' Player of the Year, Rhys Webb. Their record of winning big matches in Ireland is an impressive one, based on twice beating Leinster in Finals at the RDS.
The last four are not the only GUINNESS PRO12 teams with important weekend business. Connacht's roundabout route attempting to secure the last place in next season's European Champions' Cup, takes them to Gloucester on Sunday.
At least John Muldoon and his team know their way round Kingsholm. Having lost a Challenge Cup quarter-final there narrowly seven weeks ago, they return aiming to go one better second time round and give Gloucester more than a serious run for their money.
Form guide to the Guinness PRO12 Play-Offs - results over the last four seasons.
Warriors home to Ulster:
May 16, 2015:
Warriors 32 (Tries-Hogg, Russell 2, Vernon. Cons- Russell 3. Pens - Russell 2). Ulster 10 (Try- C Henry. Con- I Humphreys. Pen- I Humphreys).
April 28, 2014:
Warriors 27 (Tries-Maitland, Seymour, Bennett. Cons- Russell 3. Pens- Russell 2) Ulster 9 (Pens -Jackson 3)
February 22, 2013:
Warriors 20 (Tries- Swinson, Seymour, Murchie, Matawalu). Ulster 14 (Try- Olding. Pens- Pienaar 3).
October 19, 2012:
Warriors 8 (Try- Matawalu. Pen- Horne). Ulster 19 (Try- Henry. Con- Jackson. Pens - Jackson 4).
November 25, 2011:
Warriors 17 (Try- Nathan. Pens- Weir 3. Drop goal- Weir). Ulster 9 (Pens- Humphreys 3).
Munster home to Ospreys:
September 27, 2014:
Munster 14 (Try - van der Heever. Pens- Keatley 3) Ospreys 19 (Try- Hassler. Con- Biggar. Pens- Biggar 4)
November 2, 2013:
Munster 12 (Pens- Keatley 3, Hanrahan), Ospreys 6 (Pens- Morgan 2).
March 2, 2013:
Munster 13 (Try- Varley. Con- O'Gara. Pens- O'Gara 2). Ospreys 13 (Try- J Thomas. Con- Morgan. Pens- Morgan 2).
October 8, 2011:
Munster 13 (Try - Barnes. Con- Keatley. Pens- Keatley 2). Ospreys 17 (Try- Webb. Pens- Biggar 4).
The top five over the last four regular seasons of the Guinness PRO12:
P W D Lost
1 Warriors 88 63 5 20 (Try count +108)
2 Leinster 88 63 5 20 (Try count +79)
3 Ospreys 88 59 4 25
4 Ulster 88 58 3 27
5 Munster 88 56 4 28
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