An absorbing final between the world No.1 and No.2 saw Humphries lead until the third-to-last leg, as both players served up a spectacular 100-plus average and a combined nine 180s.
Legs three and four were arguably the most eye-catching as 'Cool Hand' produced a stunning 132 finish on 25, treble 19, bull, before Littler responded in typical fashion with an incredible 146 checkout.
The 19-year-old then seized an opportunity to come back from 5-4 down when Humphries missed tops for a 78 finish by taking out double 20 himself before splitting 26 and nailing double 10 to win another classic encounter.
Littler sinks MVG and Clayton
Earlier in the night, Littler played out an extraordinary quarter-final against Van Gerwen.
Beginning with arguably the worst leg of the tournament so far, with a combined 21 darts missed at double, the game soon sparked into life when the Green Machine produced world-class levels to stage a stunning fightback from 4-1 down.
The Nuke then stopped Van Gerwen in his tracks with a wonderful 141 finish on double 15, denying the Dutchman a potential 11 darter in the process, before wrapping up the game on double tops to secure a 6-5 victory after his opponent averaged 101.15 and hit five 180s.
A thrilling semi-final then followed as Littler edged Clayton 6-5, despite the Ferret hitting four 180s and posting a clinical 62 per cent (5/8) on the checkouts.
In a back-and-forth contest, the Premier League's two best players produced phenomenal darts, with the 19-year-old hitting six perfect darts twice before finishing the game on his customary double 10.
Van Gerwen's defeat gave Humphries the chance to leapfrog him into fourth position, doing so with two last-leg victories.
Starting with a nervy 6-5 win over Rock, Humphries then somehow survived three match darts against Bunting before holding his nerve on double eight to wrap up a vital win.
It was an incredible encounter in Leeds that staged some magnificent darts, including a 141 finish for Bunting, while both players averaged 105-plus and hit a combined 10 180s.
Price woes continue
Gerwyn Price, meanwhile, continued to look out of sorts in his quarter-final clash against Clayton, suffering a 6-2 loss and recording a measly 16.67 per cent on the checkouts.
As for Van Veen, he failed to take advantage of defeats for Price and Van Gerwen in his pursuit of the top four after missing two match darts in a 6-5 loss to Bunting.