Deadly Dipo Akinyemi pounced to keep Ayr's heads above water as the Championship play-off race endured another day of drama.
The striker bagged his 24th goal of a brilliant season as the Honest Men came from behind to earn a draw at Partick.
The result leaves Ayr clinging on to fourth place on goal difference - with just one point separating Thistle, United and Inverness.
Boss Lee Bullen hailed his side for claiming what could be a critical point come the shake-up.
He said: "Not many teams will come here and pass them off the pitch like we did at times. They are a really good side and have been showing it lately.
"We knew they were on a great run of form so it was important we came and stood up to that.
"So to come and put up the performance we did was great. We stayed calm when going a goal behind and showed a lot of resilience.
"It is amazing how important that kind of point becomes at this stage of the season.
"I felt it was going to be a war of attrition in the middle of the park which is why we went with the three in there to try and win that battle.
"Chrissy (Maguire) was disappointed not to start but he showed his class when coming on and when the game got stretched late on he was able to have a real impact.
"It sets us up for another massive game next Friday night against Queen's Park and they just keep coming at the moment so we need to be ready."
Bullen reintroduced Andy Murdoch to the starting XI as he went into midfield alongside Ben Dempsey and Reece McAlear.
But Ayr after a bright start in which Sam Ashford was denied early, United were flung on to the back foot as Thistle poured forward.
They should have hit the front when Scott Tiffoney blasted over but he made no mistake soon after when drilling home a Steven Lawless pass.
Ayr were under the pump in the opening quarter but slowly came through the other side and began to enjoy the bulk of possession as the first half drew to a close.
The introduction of Jayden Mitchell-Lawson and Maguire as second half subs helped to give United extra momentum as they continued to probe - with Thistle dropping deeper and deeper.
And they got their deserved goal when Maguire sent Mitchell-Lawson scampering down the right before his cross was turned home by that man Akinyemi.
Both sides threw everything at a winner in the closing stages and Ayr survived a late scare when the ball flashed across their six yard box with no Thistle takers for what would have been a sucker punch at the death.
