'The Asp' was considered an outsider before play started but got his night underway with a dominant win against Rob Cross before battling to victory against Michael van Gerwen.
An excellent comeback followed in the final against Humphries, coming from 3-0 down to win 6-4 in one of the biggest upsets of the 2025 Premier League tournament so far.
"I've had a fantastic couple of weeks and I said to my sports psychologist, I feel like I'm going to make the final," Aspinall told Sky Sports.
"I have never, ever been so nervous as them last darts. Two years ago I couldn't handle the nerves, but what a way to win it."
The win sends Aspinall into the play-off places in the Premier League and level on points with both Van Gerwen and Gerwyn Price.
The 33-year-old attributed his victory to confidence in himself, insisting that all eight Premier League players are at the same level as each other but only confidence separates them.
"All eight players are now as good as each other. There's no one better, there's no one worse than each other," Aspinall told the media after his win.
"It's who's confident at the time and I think that's what you see when people go on these runs, the confidence and the win.
"I'm confident at the moment, the last two weeks have been amazing and I can't wait to play."
Fast-paced action impresses in Manchester
Chris Dobey and Van Gerwen got the evening underway at the AO Arena in Manchester, with the Dutchman taking control to secure a 6-4 triumph after averaging an impressive 97.20 in the opening quarter-final.
Englishmen Aspinall and Cross followed and delivered another fast-paced match, with the Asp coming out on top in dominant fashion by sealing a 6-2 win.
Reigning Premier League champion Luke Littler brushed aside the experienced Price 6-2, despite his elder challenger hitting a nine-darter - the 20th in Premier League history - and five 180s.
Littler's quarter-final average of 103.93 just pipped the 103.47 of Price and allowed the 18-year-old to seal a commanding victory despite the higher scoring of the Welshman.
The openers were rounded out by Humphries and Stephen Bunting, with the former World Champion comfortably fending off last week's nightly winner by cruising to a 6-1 triumph, averaging an astounding 118.43.
In the night's opening semi-final, Aspinall faced off with Van Gerwen and produced one of the performances of the night to come out on top 6-2.
The Asp was far from one of the favourites heading into the evening but gave a good account of himself and took advantage of a sloppy showing from his Dutch opposition - with Van Gerwen having won just one of his last seven matches.
Humphries and table-topping Littler went at it in the penultimate match of the night with 'Cool Hand' getting the better of his youthful rival, triumphing 6-4.
The world number one raced into a 2-0 lead with Littler pegging him back to 4-3 - even getting one away from a nine darter - before Humphries made another push to victory and a spot in the final against Aspinall, which he ultimately lost.
Premier League Darts night 11 takes place on Thursday, April 17th, in Rotterdam.
Full results:
Chris Dobey 4-6 Michael van Gerwen
Stephen Bunting 1-6 Luke Humphries
Michael van Gerwen 2-6 Nathan Aspinall
Luke Littler 4-6 Luke Humphries
Nathan Aspinall 6-4 Luke Humphries