Contesting his first Masters final for 20 years, Williams made the ideal start with a total clearance of 138, but Trump hit back to win the next four frames in succession with the aid of breaks of 61, 106 and 73.
A second century of the match – followed by a break of 60 – saw Williams reduce his deficit to a single frame in the first clash between the pair since Trump edged an epic World Championship semi-final 17-16 last April.
However, Williams then badly misjudged at attempted double on a red to the middle pocket early in frame eight and Trump stepped in with an 89 break to stretch his lead to 5-3.
The 33-year-old needs five more frames from the evening session to win his second Masters title and pocket the first prize of £250,000.
A victory for Williams, who turns 48 in March, would make him the oldest ever Masters champion, eclipsing the record of Stuart Bingham, who was 43 years and 243 days old when he won in 2020.