Chorley ace Massaro had beaten King four times over the past three years.
But it was the latter who started on the front foot, with her accuracy and consistent hitting seeing the world No.11 build up two game balls in the opener.
Preston-based Massaro found her rhythm though and came back to win it on the tie-break.
King then struggled to replicate her start to the match in games two and three as an imperious Massaro went on to record a 12-10 11-5 11-4 win.
The victory ensured she will lock horns with either eight-time world champion and old foe Nicol David or home hero Amanda Sobhy in Saturday’s final.
“I was way down in that first one, and I think it just shows that if you’re not completely focused, then her skills are just too good, so it was about making sure that I was 100 per cent focused on the game plan that I set out before,” said world No.2 Massaro.
“Being able to stick to a game-plan is probably one of my biggest strengths and as long as I do that, I’m happy win or lose, so that’s my only focus when I’m on court.
“I’m loving my time in San Francisco, it’s an amazing event and a wonderful location to put a squash court.”