Dundee grab late equalizer in derby
John Brown's side were reduced to 10 men when captain Gary Irvine was sent off after 20 minutes, but Ryan Conroy broke the deadlock with a spectacular 67th-minute effort.
That looked like giving the SPL's bottom club a long overdue win over their rivals, until substitute Gardyne broke their hearts to keep Jackie McNamara's men in the hunt for a top-six finish.
Goalkeeper Steve Simonsen made his Dundee debut after he signed on a short-term basis to replace the injured Rab Douglas, and he was soon busy.
United began brightly and captain Jon Daly went close after three minutes with a clever flick from Barry Douglas' cross which fizzed just wide of Simonsen's right-hand post.
Gary Mackay-Steven was coming in for some heavy treatment from the Dark Blues players so it was no surprise when Jim McAlister became the first player to be booked after 18 minutes when he went in late on the talented winger.
Dundee were jolted when Irvine was shown a straight red for leading with an elbow in his aerial challenge with home midfielder Richie Ryan.
United, with their extra man, were able to dominate possession and after 31 minutes Mackay-Steven's header from Douglas' pinpoint cross came crashing back off the crossbar with Simonsen beaten.
Dundee were managing to weather the storm but had Simonsen to thank for keeping them on level terms after 36 minutes, with the former Everton and Stoke man rushing out to close the angle before getting his body in front of Mackay-Steven's shot at the expense of a corner.
In was virtually all United in terms of chances, but after 39 minutes Gary Harkins gave Dundee some hope when he almost conjured up a magical goal from outside the penalty area.
The former Kilmarnock man picked up possession 20 yards from goal and on his weaker foot, his left, got away a swerving shot which just went wide of Radoslaw Cierzniak's far post.
The second half began with a familiar pattern as the hosts did most of the pressing. Within minutes of the restart, Brian McLean rose to meet Douglas' free-kick but his header sailed over the bar.
Dundee took a shock lead midway through the half when Brian Easton's pass picked out Conroy, and with one deft touch he killed the ball before firing an unstoppable left-footed strike high into the roof of Cierzniak's net.
However, in the 89th minute United substitute Gardyne struck at the far post after Daly had headed down Stuart Armstrong's cross to send the hosts wild in celebration and leave the visitors dispirited.