A late Gary Mackay-Steven strike dented Inverness Caledonian Thistle's chances of reaching a Europa League slot and sealed an away win for Dundee United.
Billy McKay's penalty was saved by Radoslaw Cierzniak before Aaron Doran put Inverness into a 42nd-minute lead with a right-footed shot.
Ryan Dow converted Jon Daly's knock-down to level the scores after restart.
With 10 minutes remaining Doran blasted a sterling chance over the stand before Mackay-Steven's injury-time strike.
Inverness will now have to battle to secure third place in the Scottish Premier League and the valuable European football slot that comes with it next Sunday in the highland derby away to their Highland rivals Ross County, who are still within reach of the same target.
The match started brightly enough for the hosts, with Andrew Shinnie's 4th-minute shot deflected for a corner.
United hit back with a powerful drive by Stuart Armstrong in the 12th minute, but Caley keeper Reguero got a strong hand to it to block.
Brian McLean made a great recovery tackle to clear a yard from goal on 18 minutes as Inverness looked determined to find the net.
McKay should have scored from 12 yards after 33 minutes minutes, but Cierznak dived low to his right to push the ball wide.
Moments later, the home side won a penalty after John Rankin was penalised for a challenge on Doran.
McKay struck a powerful shot from the spot, but Cierznak's leap to the left was spectacularly acrobatic and he beat the ball away for a top-class save.
Inverness eventually did find the net as half-time approached when Doran, with his back to goal, swivelled to hit a sweet low shot from the edge of the box that flew past Cierznak's outstretched hand to nestle inside the right hand post.
United were back on level terms five minutes after the restart when Dow raced on to a knock-down to blast a volley low past Reguero.
Johnny Russell had a great chance to put the visitors further ahead moments later, but failed to connect with the ball 12 yards from goal.
Russell then tested the keeper with a speculative 30-yard lob, but Reguero held well under the bar.
Jon Daly headed just wide as full-time approached, but United were not to be denied, and, as the match entered stoppage time the Terrors broke away on the right and Stuart Armstrong's low cross was side footed home by Gary Mackay-Steven.
Live Text Commentary
Full Time The final whistle goes and the game is over.90:00 +2:51
Andrew Shinnie takes a shot. Comfortable save by Radoslaw Cierzniak.90:00 +2:05
Free kick awarded for a foul by Michael Gardyne on Graeme Shinnie. Free kick taken by Graeme Shinnie.90:00 +0:25
Assist on the goal came from Willo Flood.90:00 +0:25 Goal scored
Goal! - Gary Mackay-Steven - Inverness 1 - 2 Dundee U A goal is scored by Gary Mackay-Steven from close range to the bottom right corner of the goal. Inverness CT 1-2 Dundee Utd.89:57
Michael Gardyne challenges Josh Meekings unfairly and gives away a free kick. Free kick taken by Antonio Reguero.86:16 Substitution
Substitution (Inverness CT) makes a substitution, with Shane Sutherland coming on for Nick Ross.86:16
Corner taken by Willo Flood from the right by-line, Headed effort from deep inside the penalty area by Jon Daly misses to the left of the goal.85:18
Willo Flood takes a shot. Blocked by Owain Tudur Jones. Inswinging corner taken from the left by-line by Willo Flood, save by Antonio Reguero.84:06
Stuart Armstrong challenges Aaron Doran unfairly and gives away a free kick. Direct free kick taken by Graeme Shinnie.80:17
Effort on goal by Aaron Doran from deep inside the area goes harmlessly over the bar.78:36
Michael Gardyne is flagged offside by the assistant referee. Indirect free kick taken by Josh Meekings.78:36 Substitution
Substitution Owain Tudur Jones is brought on as a substitute for Ross Draper.78:36
John Rankin takes a shot. Blocked by David Raven. Inswinging corner taken from the left by-line by Willo Flood, Jon Daly produces a header from inside the area that goes over the bar.77:25
Shot from just outside the box by Stuart Armstrong goes over the bar.72:06 Substitution
Substitution (Dundee Utd) makes a substitution, with Rory Boulding coming on for Johnny Russell.72:06
Effort from just outside the area by Michael Gardyne goes over the target.70:03
Free kick awarded for a foul by John Rankin on Chris Hogg. Free kick taken by Chris Hogg.69:17
Andrew Shinnie takes a shot from just outside the penalty area which clears the crossbar.68:56
Michael Gardyne gives away a free kick for an unfair challenge on David Raven. Josh Meekings takes the direct free kick.
Indirect free kick taken by Radoslaw Cierzniak.67:40 Substitution
Substitution Michael Gardyne replaces Ryan Gauld.67:40
The referee blows for offside.66:01
Josh Meekings takes a shot from long range clearing the crossbar.65:24
Inswinging corner taken left-footed by Graeme Shinnie from the right by-line.64:42
Corner taken short by Aaron Doran. Aaron Doran takes a shot. Blocked by Stuart Armstrong.63:55 Substitution
Substitution Mark Millar is brought on as a substitute for Ryan Dow.60:52
Billy McKay is caught offside. Free kick taken by Radoslaw Cierzniak.60:02
Drilled right-footed shot by Andrew Shinnie. Blocked by Willo Flood. Graeme Shinnie takes a inswinging corner from the right by-line played to the near post, clearance by John Rankin.58:27
Johnny Russell produces a volleyed left-footed shot from 35+ yards. Save by Antonio Reguero. Free kick awarded for a foul by John Souttar on Nick Ross. Aaron Doran takes the direct free kick.57:01
Effort from the edge of the box by John Rankin goes wide of the right-hand upright.55:53
Handball by Andrew Shinnie. Brian McLean takes the direct free kick.54:57
Effort on goal by Aaron Doran from just outside the penalty area goes harmlessly over the target.54:12
The assistant referee flags for offside against Johnny Russell. Josh Meekings takes the indirect free kick.53:33 Booking
Booking Ross Draper receives a yellow card for unsporting behaviour.51:33
Free kick awarded for a foul by Willo Flood on Graeme Shinnie. Graeme Shinnie has an effort direct from the free kick.49:03
Assist by Jon Daly.49:03 Goal scored
Goal! - Ryan Dow - Inverness 1 - 1 Dundee U Ryan Dow gets on the score sheet with a goal from close range to the bottom left corner of the goal. Inverness CT 1-1 Dundee Utd.48:27
Richie Foran concedes a free kick for a foul on Stuart Armstrong. Willo Flood restarts play with the free kick. Andrew Shinnie gives away a free kick for an unfair challenge on John Rankin. Willo Flood takes the direct free kick.47:11
Richie Foran has an effort at goal from outside the box which goes wide right of the target.46:43
Aaron Doran takes a shot. Save made by Radoslaw Cierzniak.45:15
Nick Ross fouled by John Souttar, the ref awards a free kick. Graeme Shinnie takes the free kick.45:01
The second half begins.45:00 +0:21 Half time
Half Time The first half comes to an end.44:58
Free kick awarded for an unfair challenge on Ryan Dow by Richie Foran.43:01
Free kick awarded for an unfair challenge on Willo Flood by Graeme Shinnie. Free kick taken by Willo Flood. Corner taken by Ryan Dow, Jon Daly takes a shot. Save made by Antonio Reguero.41:40
Assist by Chris Hogg.41:40 Goal scored
Goal! - Aaron Doran - Inverness 1 - 0 Dundee U Aaron Doran gets on the score sheet with a fantastic goal from outside the penalty area to the bottom left corner of the goal. Inverness CT 1-0 Dundee Utd.39:55
Aaron Doran takes a shot. Willo Flood gets a block in. Inswinging corner taken left-footed by Graeme Shinnie from the right by-line. Inswinging corner taken by Aaron Doran from the left by-line, clearance made by Jon Daly.38:15
Andrew Shinnie takes a shot from a long way out clearing the bar.36:15
Andrew Shinnie produces a right-footed shot from just outside the penalty box that goes wide left of the target.35:31
Inswinging corner taken left-footed by Graeme Shinnie, clearance made by Jon Daly.34:56 Penalty
A save by Radoslaw Cierzniak from penalty by Billy McKay. Inverness CT 0-0 Dundee Utd.34:17 Booking
Booking Radoslaw Cierzniak is cautioned.33:59 Booking
Booking John Rankin is shown a yellow card.33:30
Foul by John Rankin on Aaron Doran, Penalty awarded.32:33
Billy McKay takes a shot from 20 yards. Jon Daly gets a block in. Aaron Doran takes a shot. Blocked by Brian McLean. Headed effort from inside the area by Nick Ross misses to the left of the goal.31:08
Billy McKay takes a shot. Save by Radoslaw Cierzniak. Andrew Shinnie decides to take the corner short. Stuart Armstrong gives away a free kick for an unfair challenge on Graeme Shinnie. Josh Meekings takes the direct free kick.30:44
Jon Daly challenges Andrew Shinnie unfairly and gives away a free kick. Free kick taken by Josh Meekings.29:12
Jon Daly is flagged offside by the assistant referee. Free kick taken by Antonio Reguero.28:44
Unfair challenge on Johnny Russell by Chris Hogg results in a free kick. Willo Flood takes the free kick.27:26
Gary Mackay-Steven is caught offside. Indirect free kick taken by Antonio Reguero.24:50
Ryan Gauld challenges Richie Foran unfairly and gives away a free kick. Ross Draper takes the direct free kick.22:02
Free kick awarded for a foul by John Souttar on Graeme Shinnie. Free kick crossed right-footed by Aaron Doran, Jon Daly makes a clearance.20:37
Gary Mackay-Steven is flagged offside by the assistant referee. Antonio Reguero takes the indirect free kick.18:37
Header from deep inside the area by Johnny Russell goes over the bar.17:54
Foul by Nick Ross on Willo Flood, free kick awarded. Willo Flood takes the free kick.17:22
Inswinging corner taken by Graeme Shinnie, clearance made by Ryan Dow.15:47
Willo Flood challenges Andrew Shinnie unfairly and gives away a free kick. Graeme Shinnie takes the direct free kick. Shot from a long way out by Josh Meekings goes wide of the left-hand upright.12:23
Andrew Shinnie fouled by Willo Flood, the ref awards a free kick. Free kick crossed right-footed by Aaron Doran from left wing.11:15
Stuart Armstrong takes a shot. Save by Antonio Reguero.5:24
Inswinging corner taken by Graeme Shinnie from the right by-line.3:17
Andrew Shinnie takes a shot. Blocked by Stuart Armstrong. Corner taken left-footed by Graeme Shinnie, Jon Daly makes a clearance.2:24
Shot by Richie Foran. Comfortable save by Radoslaw Cierzniak.0:00
The referee gets the game started.
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