With the score 2:5 tennis players because of the rain, left the court and resumed the match this morning.
The world number 102 said she was not "a star yet", adding she had no time to celebrate as she flies out of Hong Kong to prepare for her next tournament in Luxembourg.
She then clawed her way back from a 1-4 deficit, with Muguruza -appearing exhausted and out of ideas by the end of the match - losing her final service game with two unforced errors.
The Rome champion, who had been hoping to sew up qualification in Hong Kong for the WTA Finals, said Saturday that she was a "fighter" and had come back on court believing she still had a chance.
The Hong Kong quarter-final was a rematch of August's US Open third round contest, which the Ukrainian won in straight sets, but here the tables were turned with Svitolina receiving a tennis masterclass from her opponent.
Svitolina, the tournament's top seed, was Wang's highest-ranked top 10 scalp this year.
Wang had grabbed headlines at the French Open with her straight sets demolition of Venus Williams, and overcame Karolina Pliskova in both Beijing and Wuhan.
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Svitolina entered this week as one of five players still in contention to earn a place in the eight-player WTA Finals in Singapore.
Wang, 26, is seeking her third title and has never faced the 18-year-old Yastremska.
Next up for the Czech is second-seeded Frenchwoman Caroline Garcia, who beat Taiwanese No. 5 seed Su-Wei Hsieh 6-3, 6-4 in the other semifinal.
First career victory at the WTA tournament brought Diane Yastremsky more than 160 thousand dollars in prize money and 280 points.
Wang, the top female player from China, reached her first career semifinal at the Hong Kong Open past year.
"I was nervous in the beginning, but even though I was down 4-0, I didn't lose my fighting spirit".
Yastremska started the second set in much the same style, immediately breaking Wang with a forehand victor.