The home team qualified for the final four due to their 8-3 aggregate win over Queen's Park in the quarter-finals of the playoffs, while Ayr United are present at this stage due to their second-place finish in the Scottish Championship table.
Match preview
Partick finished fourth in this season's Scottish Championship, boasting a total of 57 points from 36 matches, just one point behind both third-place Queen's Park and second-place Ayr United.
The Jags therefore faced Queen's Park in the quarter-finals of the playoffs, and they put themselves in a position to advance courtesy of a 4-3 first-leg victory on May 9.
Kris Doolan's side then finished the job in spectacular fashion three days later, recording a 4-0 win in their away match to progress courtesy of an 8-3 aggregate success.
Partick will now face Ayr United over two legs in the hope of advancing to the final, where they would face the team that finishes 11th in the Scottish Premiership.
Thistle have been absent from Scotland's top flight since the 2017-18 campaign, and they have actually spent time in the third tier since then, so it would be a huge achievement to secure a return to the top level.
Ayr United, as mentioned, automatically qualified for the semi-finals due to their second-place finish in the table, so they have not been in action since beating Inverness Caledonian Thistle 2-1 on May 5.
Whether the long break helps them remains to be seen, but they will be aiming to secure a positive result in the first leg ahead of their home fixture against their fellow second-tier side on May 26.
The Honest Men have spent 34 seasons in Scotland's top division, but they have been absent from the top flight since the 1977-78 campaign.
Lee Bullen's side are actually winless in their last three games against Partick, including a 1-1 draw in the middle of last month, while they lost 1-0 to their semi-final opponents in February.
Partick Thistle Scottish Premiership Playoffs form:
W
W
Partick Thistle form (all competitions):
D
D
W
D
W
W
Ayr United form (all competitions):
L
D
D
D
W
W
Team News
Having not reported any fresh injury concerns ahead of the contest, it would not be a surprise to see Partick head coach Doolan select the same XI from the 4-0 win over Queen's Park last time out.
Daniel Mullen scored off the bench in the second leg of the quarter-final and is an option in a forward area, but there will once again be a starting role for Brian Graham, who has 19 goals in all competitions this term.
However, there will once again be no Stuart Bannigan for the home side, with the 30-year-old absent due to an injury that he suffered in the first leg of the last-eight clash.
As for Ayr United, a 4-4-2 formation should see Josh Mullin and Dipo Akinyemi continue in advanced areas.
Akinyemi has been in excellent form this season, scoring 24 goals in 44 appearances, while Mullin has contributed eight goals for the second-tier outfit.
There are not expected to be any real surprises in the away side's XI for this match, with Ben Dempsey, who has nine goals to his name this season, also featuring in a midfield area.
Partick Thistle possible starting lineup:
Mitchell; McMillan, Muirhead, Brownlie, Holt; Turner, Docherty; Lawless, Tiffoney, Fitzpatrick; Graham
Ayr United possible starting lineup:
Albinson; Hewitt, Musonda, McGinty, Reading; Ashford, Dempsey, Murdoch, Mullin; Akinyemi, Maguire
We say: Partick Thistle 1-1 Ayr United
This has all of the makings of a very interesting tie, and it will be fascinating to see how it plays out over the two legs. Both managers will be wary of suffering a defeat on Friday, which could create a tense affair, and we are finding it difficult to separate the two teams in the first leg.