They were on course after going ahead early but Ayr fought back to gain revenge for their midweek defeat to the Jags, leaving Gary Caldwell’s side with an anxious last-day visit to Queen of the South.
The manager said: “We played well for the opening stages, but after that we chose not to play. We chose not to run and we chose to play the way we did.
“I don’t have words to describe why that happened but we have a week to learn from it.
“It’s going to be a very difficult game on the last day of the season.
“I said to the players that they’ve just thrown a massive opportunity away.”
The game started at a high pace with chances at both ends but it was Thistle who opened the scoring when Miles Storey knocked a long ball down to Joe Cardle, who put a delicious cross in for Shea Gordon to nod in his second goal against Ayr this week following his midweek effort at Somerset Park. Cardle and Storey linked-up again on 17 minutes, but the frontman, who came in to replace Scott McDonald, could only hit the side-netting after finding room inside the box.
Ayr levelled after great work from midfielder Andy Murdoch. The 24-year-old danced down the left and teased a low ball across the box, which was knocked in by Liam Smith.
Teenager Luke McCowan, handed his first league start by Ayr boss Ian McCall, broke free from the Thistle defence on 40 minutes but his shot was blocked by Steven Anderson who raced back to the rescue.
Ayr deservedly took the lead when Declan McDaid’s fine corner was met on 12 yards by former Thistle defender Aaron Muirhead who lashed a volley into the top right-hand corner.
Thistle made three changes in quick succession after the hour and striker Kris Doolan, pictured, who was honoured pre-match for his 400th appearance, had a chance to level but his curled effort was saved by the impressive Ross Doohan.
The Jags went even closer in the dying seconds when Stuart Bannnigan’s looping cross was headed towards goal by substitute Lewis Mansell, with the back post keeping the ball out the net.
Afterwards, United boss McCall said: “I thought we did well. The pitch suits us and I was pleased with the performance.”