The Swans suffered a cruel defeat at promotion rivals Forest to miss the chance to narrow the gap on the City Ground outfit.
The killer blow came a minute into injury-time when Luke Chambers headed home a Lewis McGugan corner.
Swansea rarely threatened Lee Camp in the Forest goal but they felt aggrieved with 15 minutes left to play when Darren Pratley felt he should have had a penalty after going down under goalscorer Chambers' challenge.
Darren Pratley was fit enough to start for the Swans at the City Ground.
The midfielder was forced off early on against Peterborough seven days earlier with a knee injury but was passed fit to play in this table-topping clash.
Mark Gower replaced Ferrie Bodde in midfield while David Cotterill missed out with a hamstring injury, Tom Butler came in for him.
Second-year pro James Burgin, who hails from nearby Derby, provided the defensive cover on the bench and there was also a place for youth team midfield product Lee Lucas.
Cedric van der Gun (hamstring), too, was back from a nine-week lay-off to take his place amongst the substitutes.
Forest: Lee Camp (C), Chris Gunter, Wes Morgan, Luke Chambers, Chris Cohen, Paul Anderson (Garath McCleary, 84'), Paul McKenna (Lewis McGugan, 7'), George Boyd, Guy Moussi, Radoslaw Majewski (David McGoldrick, 77'), Dexter Blackstock.
Not Used: Paul Smith (GK), James Perch, Joe Garner, Dele Adebola.
Swans:Dorus de Vries; Angel Rangel, Ashley Williams, Alan Tate(C), Jazz Richards; Leon Britton, Mark Gower, Darren Pratley, Tom Butler (Craig Beattie, 85'), Nathan Dyer (Cedric van der Gun, 46'); Shefki Kuqi.
Subs:David Cornell (GK), Lee Trundle, Guillem Bauza, James Burgin, Lee Lucas.
Attendance: 25,012 Away: 1,544
Referee: C Webster (Tyne & Wear)
A minutes applause for Nottingham-born Keith Alexander, the Macclesfield manager who sadly passed away this week, got this highly anticipated table-topping affair under way and within the opening moments it certainly was an explosive affair.
Debutant George Boyd screwed an angled shot into Dorus de Vries' side netting with just 50 seconds on the clock and within a minute the action switched to the other end where Darren Pratley fired across goal and past Lee Camp's goal.
Dexeter Blackstock, who was at QPR during the same time as Paulo Sousa, saw a third minute goal ruled out for offside while Angel Rangel forced Camp into saving the game's first attempt on target barely a minute later.
The Swans saw another goalbound attempt blocked on six minutes, this time from Mark Gower inside the area, but the home side were to suffer a huge blow within seconds as skipper Paul McKenna was forced to withdraw through injury.
After a non-stop start to this game things started to quieten down as it reached the midway point - Boyd rifled a shot just past the top of Dorus de Vries goalframe while, after a series of Swansea corners, Shefki Kuqi had the best chance so far when he capitalised on a Wes Morgan mistake inside his own area.
Finnish striker Kuqi was racing onto the end of a long punt forward and, after Morgan misjudged the bounce, he stole possession and forced Camp into using his body to stop his attempted flick.
Dorus was called into action to make an excellent save of his own on 27 minutes - the ball ping-ponged between both sets of players in and around the area only for Boyd to finish it off with a stunning strike from 16-yards which the Dutch 'keeper spectacularly stopped with his legs, deflecting the ball away to safety.
Things were now starting to intensify and with 11 minutes before the break it was a former Swan that almost created an opening for a Forest opener.
Paul Anderson, who helped the Swans to League One success two seasons ago, tormented third tier defences with his blistering pace back and he was at it again today, this time flying past Jazz Richards and delivering a pin-point cross which was met by stand-in left-back Chris Cohen and struck powerfully at goal but, fortunately, ended up in the stand.
Both sides continued to threaten in the closing stages but were unable to muster any real chances before going in at the break.
Half-Time: Forest 0 Swans 0
The Swans made an encouraging start to this second period, which saw the introduction of Cedric van der Gun for the first time since early January, as Kuqi burst from midfield unchallenged only to send his low effort across goal and wide from 20-yards out.
They continued to look for the opener but Polish international Radoslaw Majewski had a great chance to break the deadlock eight minutes in when he connected with Chris Gunter's cross only to fire wide of goal.
Forest substitute McGugan had a chance of his own five minutes later when he completed a fine sweeping move into the Swansea area only for de Vries to race out to collect the ball at the midfielder's feet.
There werea few grumbles eminating from the stands at the City Ground as possession was being lost to their opponents, and their frustration could have grown on 66 minutes as Kuqi was able to reclaim the ball right inside Forest's six-yard box, pulling it right across the face of goal where Darren Pratley was inches away from converting into an empty net.
Cedric van der Gun saw a shot blocked on the edge of the area while de Vries was forced into making stops from Boyd and Cohen as the game headed into its final quarter.
The Swans felt they should have been awarded a penalty with a little over quarter of an houre remaining - van der Gun played a fine ball into Darren Pratley's path and as he entered the area went down under the challenge on Luke Chambers. The referee looked over at it assistant, he kept his flag down meaning that nothing was awarded to the bitter disappointment of players, management and fans.
Both sides continued to press and as the game entered injury-time the Swans thought that they might have done enough to get the point, but football can be a cruel game indeed and, as we entered added time, Forest snatched the winner.
It came from a Lewis McGugan corner, floated over the top of de Vries to the back post where Luke Chambers was placed to head into the net from close-range.