David Turnbull fired his second goal in as many games as Motherwell moved 14 points clear of the relegation spots with a 1-0 smash-and-grab win over Dundee.
Jim McIntyre's Dark Blues could have climbed out of the drop zone had they backed up Wednesday's win over Hearts.
But the remain 11th, just a point above basement boys St Mirren after Turnbull followed up his midweek winner against Hibernian with another precious goal from the penalty spot.
Well remain ninth but their victory at the Kilmac Stadium will be another massive relief to boss Stephen Robinson as they pull away from the scrap at the bottom.
Especially as the Fir Park side found themselves outplayed during the first period.
Motherwell had claimed the spoils on their last four trips to Dens Park but it was the hosts who created chance after chance.
Andy Dales wasted the first opening of the afternoon 18 minutes in. Craig Curran robbed Liam Grimshaw before releasing the on-loan Scunthorpe winger, who pushed his effort wide.
Well's only first-half chance came on 27 minutes as the ball broke to Curtis Main in the box. The the big striker turned well to get a shot away but saw it creep wide.
Andy Nelson was next to threaten for the hosts as he drove through the middle but took the wrong option as he fired over when Deacon was waiting in space to be played in.
And the visitors were grateful as Dundee again failed to punish a mistake on the stroke of half-time. Tom Aldred let the ball get stuck under his feet but Neilson could not take advantage as he stole possession before dragging wide with just Mark Gillespie to beat.
Dundee kept the pressure on as the game got back underway, with defender Ryan Inniss sending a header inches wide from a brilliant Nathan Ralph delivery.
Stephen Robinson knew he had to make a change to inject some life into his side. He threw on Elliot Frear for Gboly Ariyibi and his introduction led to the breakthrough within six minutes.
The striker seemed to be going nowhere as his picked the ball up wide in the penalty area but on-loan Crystal Palace defender Inness dived in to a silly challenge.
Referee Willie Collum had no hesitation in awarding the spot-kick that Turnbull rammed home for the opener on the hour mark.
Frear came close to adding a second as he fired across the face of goal while Deacon saw a 20-yard effort fizz over for the hosts.
The Englishman got his next try from a tight angle on target but Gillespie stood up well to parry over.
Dales put the winger in again moments later but Deacon drove inside only to find himself crowded out by Aldred, much to the frustration of the fed-up Dundee faithful.
ga('create', 'UA-72310761-1', 'auto', {'name': 'pacontentapi'});
ga('pacontentapi.set', 'referrer', location.origin);
ga('pacontentapi.set', 'dimension1', 'By Andy Newport, Press Association Sport');
ga('pacontentapi.set', 'dimension2', '84db9d02-a856-4542-8b77-ebe3639a6985');
ga('pacontentapi.set', 'dimension3', 'paservice:sport,paservice:sport:club-news,paservice:sport:football,paservice:sport:match-reports');
ga('pacontentapi.set', 'dimension6', 'story');
ga('pacontentapi.set', 'dimension7', 'composite');
ga('pacontentapi.set', 'dimension8', null);
ga('pacontentapi.set', 'dimension9', 'sport:football');
ga('pacontentapi.send', 'pageview', { 'location': location.href, 'page': (location.pathname + location.search + location.hash), 'title': 'Turnbull turns the screw on Dundee'});
