PHOTO: AFP
The 19-year-old's blitzed world number 20 Donna Vekic in the quarter-finals of the Memorial Drive tournament and was a set up against former Australian Open winner Angelique Kerber when the German retired from their final 16 match with back pain.
Yastremska, ranked 24th, will play for the fourth title of her career on Saturday against either Australian world number one Ashleigh Barty or American Danielle Collins.
"I'm pretty happy, this new season is off to a good start," Yastremska said after defeating Sabalenka of Belarus for their third match in as many meetings. "I'm just doing my job on court, playing my game. I do what I can on court. I feel confidence heading to the Australian Open."
Yastremska broke once per set while losing her own service once in the 106-minute showdown.
She finished on a third match point in the tiebreaker as Sabalenka hit the net on a return.
Yastremska claimed two WTA trophies last season at Strasbourg and Hua Hin, Thailand.
In the men's competition, South African Lloyd Harris knocked out fellow qualifier Tommy Paul 6-4, 6-7 (3/7), 6-3 after coming through on his third match point.
The 22-year-old will play his first ATP career final against either second seed Felix Auger-Aliassime or number three Andrey Rublev.
"To be honest, it doesn't seem real now," Harris said. "It was a tough match for both of us, we were both nervous.
"I'm really, really happy with the win. I had to go for it when I was serving for the match, he was ready to make me play," he added.
"I knew it would be tough to serve it out, I had to put everything I had into the end."
Harris needed two hours and 11 minutes to advance over the American, firing 13 aces and breaking four times while losing his own serve twice.
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