The new street-wise and clinical Dundee United took a break at Firhill.
And in its place returned the Dundee United that fritters away glorious chances of victory.
Mixu Paatelainen’s men were utterly dominant in the first half and squandered half a dozen clear-cut openings.
And Partick Thistle were able to land the sucker punch on the Tangerines in the second half, as so many other sides have been able to do this season.
The game in hand has now gone and the gap to Kilmarnock is five points.
If relegation is to be United’s fate, this game will go down as a big momentum changer.
For so long this looked as if it would be the night they got themselves to within two points of the play-off place.
Ryan Dow was in excellent form against St Johnstone at the weekend and he looked in the mood at the start of this contest as well.
Just two minutes into the game he found space between the Thistle backline and midfield and drew a foul 25 yards from goal.
Paul Paton took the free-kick but fired it well over the bar.
Dow should have scored a few minutes later.
Henri Anier was released down the right wing and his cross into the danger area was cushioned into Dow’s path by Billy Mckay.
The weekend Tayside derby matchwinner struck a low first time shot but it was too close to Tomas Cerny, who saved.
It would have been an excellent goal, but a controversial one because Partick’s Gary Fraser was lying on the ground waiting for the referee to stop play while United were breaking up the other end.
When Andrew Dallas did halt the match moments later, Fraser got his treatment, but it was to no avail and he was replaced on 12 minutes by Mathias Pogba.
As was the case for Saints on Saturday, the Thistle players were struggling to pin Dow down. He was finding pockets of space wherever he roamed and on 19 minutes it was on the left-hand side of the pitch. From there the 24-year-old cut inside and tried his luck with a shot that was at a comfortable height for Cerny to deal with.
By the midway stage of the first half Dow could have had a hat-trick, and his third sight of goal was probably the best chance of the lot. Around 10 yards out and with time and space he skied the ball over the bar.
Eiji Kawashima had been a virtual spectator for the best part of half an hour but when he was eventually called into action he was alive to a driven Callum Booth cross to tip it away from two waiting Thistle players.
It was soon back to the story of United spurning good chances. For a change Dow wasn’t the guilty party when Cerny was caught in no man’s land. Mckay was the player unable to keep a hooked shot from the edge of the box on target.
John Rankin was next to try his luck with a fierce left foot shot that Cerny tipped over the bar. This one had to be put down as a great save rather than a miss.
Just before the break United did find the net but Mckay’s effort was disallowed, with the man who crossed to him, Rankin, deemed to be offside.
There was a change in momentum after the interval, Thistle looking like a different team at the start of the second half.
For the first time in the match they put together a period of sustained pressure. Pogba came close with a back post header and Booth with a looping volley.
United were able to weather this mini-storm and had a near miss of their own just after the hour when a Gavin Gunning shot from 20 yards took a slight deflection and scraped past the post.
The visitors made their first substitution on 67 minutes, Guy Demel being replaced by Blair Spittal.
With all the chances created, this game was never going to be a 0-0 and it was Thistle who broke the deadlock with a brilliant goal.
The ball was crossed over from the right, David Amoo dummied and Kris Doolan lashed a low 18-yarder home.
United weren’t able to fashion any late opportunities and Kawashima was the busier keeper as the clock ran down.
And in stoppage time substitute Scott Fraser was booked for diving in an attempt to win a penalty.
Partick Thistle Cerny, Miller, Seaborne, Welsh, Booth, Osman, Amoo, Doolan (Edwards 81), Lawless, Frans (Dumbuya 47), Fraser (Pogba 13). Subs not used Scully, Elliot, Nesbitt, Nisbet.
Dundee United Kawashima, Dixon (Ofere 79), Morris, Paton, Mckay, Rankin, Knoyle, Dow (Fraser 79), Gunning, Anier, Demel (Spittal 67). Subs not used Szromnik, Dillon, Murray, Durnan.
Referee Andrew Dallas.