Devante Cole rescued a worthy point for Motherwell after a stirring second-half fight-back at Tannadice.
Goals from Lawrence Shankland and Archie Meekison had the Arabs seemingly home and dry by the interval and they even had the luxury of a missed penalty by Shankland.
Christopher long pulled a goal back early in the second half and for a long time it looked like that would be it, until Cole popped up inside stoppage time to fire in a superb volley for his 12th goal of the season.
Motherwell hadn't offered much in the first half, with Long firing wide and Ricki Lamie seeing a shot tipped over the bar by Ross Doohan as their only real chances.
The visitors stepped it up in the second half and Long forced a goal back in 53 minutes when his shot was deflected in via United defender Mark Connolly.
But though Jake Hastie made some good forays forward, all too often Motherwell were left frustrated as they failed to make an impact on the United back line.
However, Lamie flicked the ball on and Cole slammed in a superb volley to level.
United boss Micky Mellon made eight changes from the side that lost to Hibs in the Scottish Cup semi-finals. On-loan Celtic goalkeeper Ross Doohan made his debut and there were starts for Adrian Sporle, Mark Connolly, Peter Pawlett, Logan Chalmers, Archie Meekison, Louis Appere and Kerr Smith came in for (GK) Deniz, Jamie Robson, Ryan Edwards, Mark Reynolds, Callum Butcher, Ian Harkes, Mark McNulty and Nicky Clark.
For clarity, only Liam Smith, Jeandro Fuchs and Lawrence Shankland stayed in the side.
Motherwell manager Graham Alexander made just one change, with Aberdeen-bound Declan Gallagher replaced by Tyler Magloire.
Goalkeeper Scott Fox made the bench for this game, having missed almost the entire season through injury.
Motherwell were dealt a blow five minutes in when Allan Campbell slipped on the turf and rolled an ankle, before limping off.
United twice nearly took the lead while Well were down to 10 men, with Shankland latching onto Appere's pass but seeing his shot well blocked by Liam Kelly, before Meekison's curling effort was cleared off the line by Ricki Lamie.
Robbie Crawford then replaced Campbell two minutes later.
United took the lead in 13 minutes when Chalmers picked up possession down the right and his cross was headed in by Shankland into the right corner.
The hosts were given a chance to double their lead from the spot in 19 minutes when Shankland darted in front of Magloire in the box and went down, with referee David Dickinson pointing to the spot.
Shankland stepped up, but fired wide of the left post.
Motherwell didn't offer much but Long went close with a shot in 26 minutes that bounced past the right post, with Doohan scrambling to meet it.
Shankland had another chance to grab his second of the evening, but Magloire did enough to put the Scotland striker off and he blasted past the left post.
United did double their lead though in 33 minutes when Chalmers set up 19-year-old Meekison, who blasted into the left corner from just inside the box.
The best Motherwell could muster at that point was in 37 minutes when O'Donnell raced forward and drove a pass over the face of goal, but there were no takers.
But the visitors created a much better chance two minutes later when Long's cross from the left was met by a flick from Lamie, but Doohan did well to tip it over the bar.
Motherwell's Jake Hastie tried his luck from distance in 43 minutes, but his shot sailed well over.
Chalmers's corner was headed past by Connolly in 44 minutes as United tried to stretch their lead ahead of the interval.
United nearly stretched their lead further four minutes into the second half when Liam Smith played Shankland into the box and his neat back-heel flick returned the favour, but the defender's low shot was brilliantly punched past the left corner.
However, Motherwell came roaring back into the game in 53 minutes when O'Donnell's throw-in from the right was played on by Devante Cole to Long, and his shot was deflected past Doohan by defender Connolly.
Nathan McGinley played a superb ball to Long on the hour-mark, but Doohan saved brilliantly as the offside flag went up on the far side.
Cole then set up Hastie for a shot in 76 minutes, but his effort was well blocked.
It looked like that would be it for the Steelmen, but inside stoppage time, keeper Kelly's free kick was flicked on by Lamie and Cole flashed an unstoppable volley past Doohan to snatch a point.
DUNDEE UNITED: Doohan, Sporle, Connolly, K. Smith, L. Smith, Fuchs, Pawlett (Harkes, 62), Meekison (Butcher, 69), Chalmers (Robson, 81), Appere (McNulty, 81), Shankland.
Subs (not used): Deniz (GK), Reynolds, Clark, Edwards, Davidson.
Booked: Butcher (85).
MOTHERWELL: Kelly, O'Donnell, Magloire, Lamie, McGinley, Foley (O'Hara, 79), Campbell (Crawford, 7), Lawless (Watt, 64), Hastie, Long, Cole.
Subs (not used): Fox (GK), Carroll, Maguire, Dunne, Cornelius, Galalgher.
Booked: Long (30), Cole (58), Lamie (90+7).
Referee: David Dickinson.
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