Dundee United fell to the foot of the Scottish Premiership table as "terrific" relegation rivals Kilmarnock picked up a vital, narrow win.
Steven Fletcher had the chance to give United the lead from the spot, but the veteran striker saw his penalty well saved by Sam Walker.
Minutes later, Kilmarnock made them pay as Danny Armstrong lashed in a first-time effort from 12 yards out.
Derek McInnes' side move three points off the bottom, jumping to ninth place.
"We were nervy at the start, but once we hit our stride, we were knocking on the door, asking the question, being really aggressive and moving the ball," said McInnes.
"Tonight it was a really strong performance. I'm just disappointed we didn't make the most of our opportunities and we couldn't relax and enjoy the game more."
A lot of pre-game talk centred around where Kilmarnock's goals would come from following the departure of Kyle Lafferty and Oli Shaw on deadline day.
Between them, that duo had scored one league goal all season. Armstrong had scored five before kick-off. He'd be influential again.
Before that though, Fletcher missed a glorious chance to help United to their first league win at Rugby Park since May 2016.
Scott McMann had his shirt pulled by Armstrong and fell to the floor. Fletcher stepped up, but the recalled Walker made a fine save.
Armstrong made amends 10 minutes later. Luke Chambers, on his debut after signing on loan from Liverpool, sent in a low inviting cross that Armstrong lashed high into the net with a first-time strike.
Until the final 20 minutes, Kilmarnock looked the more likely to score again. Kyle Vassell had two superb opportunities but fluffed his lines, and Ryan Edwards almost put the ball into his own net, but Liam Smith made a vital block.
Then, from nothing, United's fightback appeared. Glenn Middleton placed a volley inches past; Fletcher fired over the bar after cutting onto his left; and 16-year-old Rory MacLeod dragged a shot centimetres across the goal.
It wasn't enough. The missed penalty came back to bite them. Back at the bottom, it could be a costly mistake.
Like much of this season, the little winger was at the heart of everything good Kilmarnock did.
Kilmarnock could have made this victory a lot more comfortable. Vassell missed chances and Mark Birighitti had to produce a few decent saves, too.
While McInnes admitted he was disappointed to lose Lafferty, losing Armstrong would have been catastrophic. Thankfully for the manager, they've kept the man most likely to keep them up.
United offered very little with the exception of the final stages. They simply need to turn up for the full 90 - or even 45 minutes.
A significant win for Kilmarnock, and a significant loss for United.
Kilmarnock goalscorer Danny Armstrong: "It was huge. We all know how big that win was. Hopefully it's the start of a wee run and it gives the boys a lift, because it was a deflating week last week with the loss to Ross County.
"I was disappointed in myself giving away the penalty. I just thought, 'I want to redeem myself'."
Dundee United manager Liam Fox: "It's a tough night for us. We were poor, flat from the very start. Lots to work on.
"They're down. They feel like they've let themselves down, the club and the fans too. But no one is relegated tonight. We get back to work. I believe in the group, I've said that from the start."
Kilmarnock travel to Livingston on Saturday, while Dundee United visit Tynecastle to face Hearts (both 15:00 GMT).