After the Millbrae men won the RBS Premiership a week ago their coaches Kenny Murray and Peter Laverie were at pains to point out the team would not be letting their foot off the gas.
They are now just one step away from Murrayfield – where they won in 2010 and 2011 and were runners-up last term – after this latest win.
County, fresh from their own impressive league win over Gala, started sharply at Millbrae and took a 7-0 lead, thanks to a converted try.
Ayr's Finn Russell then kicked a penalty before a County drop goal extended their lead to 10-3. Before the interval the home side squared things up with a converted try from Andrew Dunlop.
Early in the second half County regained the lead with a penalty before Ayr cranked things up a gear and scored 21 unanswered points to make it through to the last four. Richard Dalgleish scored a converted try to make it 17-13 and then man-of-the-match Craig Gossman bagged a double, with Finn Russell converting both.
Ayr will be joined in the draw by Gala after last year's champions won 27-17 against Aberdeen Grammar at Rubislaw. Gala went 10-0 up in the first quarter, thanks to a Craig Robertson try and five points from the boot of London Scottish-bound Lee Millar.
However, by half-time they led by just two points at 13-11 and had Euan Dods in the sin-bin.
A drop goal and a penalty put the home side 17-13 up before Grant Somerville's converted try put Gala back in front at 20-17. A penalty try easily converted by Millar then sealed it for the Borderers.
The other two teams to make the semis are Dundee HSFP and the 2008, 2009, 2010 and 2011 finalists Melrose. Dundee edged out Currie 30-29 at Malleny Park, holding on despite a late comeback from the hosts, and Melrose won 27-13 at Borders rivals Selkirk.
Meanwhile, Boroughmuir moved a step closer to relegation from the RBS Premiership when a late try by Colin Goudie saw Heriot's sneak a 23-22 win at Meggetland.
'Muir looked to be up for this one from the off and by the 44th minute led 22-3, their points coming from two tries by Ross Bradford (2) and one by Stevie Wilson, and a penalty and two conversions by Ally Warnock.
Jonny Latta was then sin-binned and while he was off Heriot's came back to 22-15 before a Graham Wilson penalty and the dramatic try saw Heriot's home.
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