A hotly-disputed second half penalty saw Hearts become the first team to beat Ayr at Somerset Park this season.
United skipper Jack Baird was penalised just after the restart when a cross from Andy Halliday bounced up off his thigh and struck his arm.
But Ayr were incensed by the award from whistler Steven McLean, with Liam Boyce sticking away the spot-kick.
It proved the pivotal moment on a night which had initially promised much for Ayr, with Mark Kerr's men on the front foot against the odds-on title favourites.
Kerr had stuck with the side that trounced Alloa 4-1 last week and they looked in confident shape during the early stages.
However, they did have to survive an early penalty shout of their own with the game just five minutes old when Boyce saw his run into the box checked by Corrie Ndaba, with the loud appeals were waved away.
But United showed they were more than up for a contest and they should have been in front with the game 15 minutes old.
A poor kick out by Craig Gordon went straight to Tom Walsh and when he centred, Andre Wright couldn't get the ball out from under his feet as the chance was lost.
Ayr continued to press and were on the hunt again in the 23rd minute when Cammy Smith broke from midfield but his left foot drive from just outside the box drifted just wide.
It was the hosts who looked the likelier in the first half and frustrations were boiling over from the visitors, with boss Robbie Neilson receiving a yellow card for dissent on the touchline.
But they re-emerged for the second half and got the perfect start with the penalty award which Boyce tucked away with authority.
That gave the Jambos some breathing space and they were able to take a grip of the ball as Ayr chased the game.
Neilson introduced new signings Aaron McEneff and Gervane Kastaneer as the game entered its final quarter.
And there was also a debut for Ayr's new loan signing Brett McGavin in the centre of the park.
Ayr also threw on Michael Moffat and Mark McKenzie in a bid to find the leveller but Hearts saw it out for another vital win on their relentless charge at the top.
