Williams, 38, edged the first set with a timely break at 5-4 and broke again early in the second before serving up a masterclass with the loss of just five more points on serve on her way to victory.
A day after knocking out world No 3 Caroline Wozniacki of Denmark, 18-year-old Andreescu put on another stunning display to oust 2015 Auckland champion Williams. But I'm honestly really excited just being here.
She blasted the 20-year-old Kuzmova off the court in the first set, with a number of winners and unreturnable serves.
The unseeded Viktoria Kuzmova of Slovakia faces No. 2 seed Julia Goerges in the other semfinal after the German downed Canada's Eugenie Bouchard 3-6, 6-3, 7-6 (6) in a quarterfinal.
Goerges is seeded No. 2 and was last year's champion.
"I believe that anything is possible and tonight I think I did the impossible, " Andreescu said.
Andreescu says she doesn't know if she's in a dream right now or not.
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She beat top-seeded Wozniacki 6-4, 6-4 and sixth-seeded Williams 6-7 (7), 6-1, 6-3.
Andreescu's win over Hsieh in a semifinal Saturday was her most dominant performance of the week.
The second set ended up going to a tiebreak.
Standing in the way of Goerges and a second straight Auckland crown is giant-killer Andreescu. "I was still aggressive, but not aggressive as much as I wanted to be". Andreescu had to earn her way into the ASB Classic by winning the qualifiers. Starting off the new tennis season, Andreescu has made diversifying her game a priority.
"Everybody deserves their spot in the final, so she has played some terrific tennis", Goerges said.
"You have to start somewhere and I guess this is my awakening moment, maybe", she said.
Anisimova, ranked 96th in the world and the youngest player in the top 100, never dropped a game against the Czech veteran and cashed in on all three break points that went her way.