Top seed Sabalenka stunned by Muchova in Beijing last eight
The Czech player won 7-6 (7/5), 2-6, 6-4 and faces China's Olympic champion Zheng Qinwen or 17-year-old Russian Mirra Andreeva for a place in the final.
Last year's US Open champion Coco Gauff plays Spain's former world number two Paula Badosa in the other semi-final in Beijing.
World number two Sabalenka was on a streak of 15 victories in a row, claiming titles in Cincinnati and then the US Open for the first time.
The Belarusian previously had trouble though with Muchova, who this time last year was inside the top 10 before injury struck.
US Open semi-finalist Muchova had won the last two meetings between them with both going to a deciding set.
This encounter proved to be just as tight as Sabalenka's hopes of usurping Iga Swiatek at the top of the rankings took an unexpected hit, having been a break up at 4-2 in the deciding set and seemingly on course for the last four.
"The conditions here are so tough, it can go one way or the other. There was a couple of great returns from her, she got the momentum and started playing more aggressively," said Sabalenka, who will play at the Wuhan Open next week.
"It was a really high-level match and she played some unbelievable tennis."
Three-time major champion Sabalenka faced three break points at the start but she held firm, then had a sniff herself on her opponent's serve at 2-1, but Muchova similarly refused to buckle.
The hard-hitting Sabalenka had set point on Muchova's serve at 5-4 but surrendered the opportunity when she rattled her forehand wide on the Czech's second serve.
Muchova double-faulted to give Sabalenka another chance to wrap up the first set but again she failed to take it.
The Czech had two set points in the tiebreak and grabbed the second at the end of a thrilling rally to go a set up.
It was the first set 26-year-old Sabalenka had dropped since the third round in New York.
Sabalenka started the second set brightly to break for 2-1 and broke once more on the way to sealing the set with comparative ease.
Neither could hold serve in the deciding set before Muchova, who at one point had appeared to be wilting in the face of her opponent's superior power, found another gear to clinch the match in two hours, 46 minutes.
This time last year 28-year-old Muchova was ranked ninth in the world having reached the French Open final and semi-finals at Flushing Meadows.
But her 2023 season ended after the US Open because of wrist surgery and she did not play for nearly 10 months until returning this summer.
Sabalenka felt she should have been more aggressive in the match, but said: "I was really close to winning today, but things didn't go my way and I can only be proud of myself," she said.
