Having returned this year after the birth of her first child, Williams made the Wimbledon final in July but lost to Angelique Kerber.
The veteran American had also been beaten in her previous two semi-finals at Flushing Meadows, in 2015 and 2016, but is now on the brink of glory in New York following a sensational display on a stormy night.
So impressive throughout this tournament, Williams looks to be back to her brilliant best and again delighted her many supporters on Arthur Ashe Stadium.
Sevastova's use of drop shots had proven highly effective against defending champion Sloane Stephens in the previous round, but the Latvian last-four debutant was overwhelmed on Thursday as the ruthless Williams charged to the net at every given opportunity and put away winner after winner.
Williams, who turns 37 later this month, is the third-oldest female slam finalist in the Open era, behind Martina Navratilova (Wimbledon 1994) and her sister, Venus (2017 Wimbledon).
Navratilova and Venus both went on to lose their finals, but six-time US Open champion Serena will undoubtedly start as a heavy favourite when she faces Madison Keys - last year's runner-up - or Naomi Osaka on Saturday.
There was an uneasy hush in the arena, which had its roof closed due to the weather, as Williams started sluggishly and was broken in the first game of the match.
Sevastova looked impressively assured on the big stage early on, but her signature drop shot let her down as she allowed her improving opponent to draw level at 2-2.
While both players were making too many errors on the forehand wing, there was certainly no problem with Williams' backhand as she pulled off a glorious pass in the next game.
Williams' level was rising ominously and she wrapped up the set in quick time thereafter, frequently coming forward to great effect and also enjoying notable success attacking the second serve of Sevastova.
Those themes continued in a totally one-sided second set, Serena charging to the winning post as only she can.