Muchova stuns Sabalenka to set up Beijing semi with home hero Zheng
The unseeded Muchova beat the world number two 7-6 (7/5), 2-6, 6-4 in Beijing and said: "Tennis is a roller-coaster."
In another topsy-turvy quarter-final, home hero Zheng fought back to defeat Mirra Andreeva 5-7, 6-0, 6-4, with the 17-year-old Russian reduced to tears in the third set.
Last year's US Open champion Coco Gauff plays Spain's former world number two Paula Badosa in the other semi-final.
Sabalenka's streak of victories saw her claim titles in Cincinnati and then the US Open for the first time.
The Belarusian previously had trouble though with Muchova, the Czech who is now ranked 49 but 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 third set.
This encounter proved to be just as nervy as Sabalenka went out despite having been up 4-2 in the deciding set and seemingly on course for the semi-finals.
"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 had set point on Muchova's serve at 5-4 up in the first set 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.
Sabalenka started the second brightly to break for 2-1 and broke once more on the way to sealing the set with comparative ease.
Both struggled on 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.
Andreeva in tears
In front of an expectant packed house at the 15,000-capacity Diamond Court, Andreeva silenced the home crowd and fought off a comeback from Zheng to win the first set.
The second set was all Zheng, the world number seven who is playing her first tournament back home in China since winning Paris gold.
The occasion seemed to get to the teenage Russian, who had tears in her eyes as she served at the start of the deciding set.
At 3-2 up the 22nd-ranked Russian had a medical timeout and was tearful again as she had strapping applied to her left wrist.
She recovered her poise for 4-2 but Zheng rattled off three games in a row and served for the match, sealing the triumph on first match point when Andreeva's return drifted beyond the baseline.
After reaching her first semi-final in the Chinese capital, the 21-year-old Zheng said she had played only "20-30 percent of my level.
"I told myself: I am not going to win today by my tennis. I am going to win by my mentality."
She was not aware of the emotions of her opponent, but said: "I cry a thousand times every time when I had a chance (and did not take it).
"But I don't usually cry during the match. I cry after the match."