For Smith, it is a first final in the tournament and a potential opportunity to claim a first TV major title when he faces either Peter Wright or Gary Anderson.
Aspinall started the contest superbly going 4-2 up after initially doubling his lead in the early going and this was extended with a brilliant break of throw with a 60 checkout after Smith fluffed his lines on a setup shot to make it 6-2.
Despite that Smith came back and after being three legs behind, it was back to within one at 5-4. Aspinall kept him at arms length though at the next interval making it 6-4 as the game became really close.
A turning point came up though with Aspinall squandering six darts to hold at 6-5 up and Smith hit double 10 to level and from there Smith took the lead.
At the interval he was 8-7 up after four of the previous five legs went his way and he looked good in this nail biter.
It continued to be nip and tuck and went all the way to a decider after Aspinall held his throw but Smith held his to claim victory and move through to the final.
Click here for more information on The Masters, the traditional season opener in Milton Keynes.
Evening Session (1900 GMT)
Semi-Finals
Nathan Aspinall (92.4) 10-11 (92.48) Michael Smith
Peter Wright v Gary Anderson
Final
Michael Smith v Wright/Anderson
https://twitter.com/OfficialPDC/status/1224061081938792449?s=20