The 26-year-old De Minaur led 4-1 in the first set of Monday's quarter-final when Czech 20-year-old Mensik had to withdraw due to a problem with his left leg.
De Minaur will now play Sinner after the Italian beat Hungarian Fabian Marozsan 6-1 7-5 to narrow the gap on world No.1 Carlos Alcaraz.
The man known as "Demon" will go in as a pronounced underdog. The world No.8 is yet to beat Sinner in 10 meetings, the latest a 6-3 6-2 6-1 trouncing at this year's Australian Open.
Sinner lost the top ranking after surrendering his US Open crown to Spaniard Alcaraz this month but has responded well in Beijing, claiming a seventh semi-final spot from eight tournaments played this year.
He clinched the opening set in 26 minutes thanks to some big hitting from the baseline but faced much stiffer resistance in the second and had to dust himself off after falling awkwardly in the eighth game.
Sinner then found himself a break down at 4-5 but hit back immediately to deny Marozsan the set.
The 24-year-old Sinner held, then pounced on his opponent's serve again in the 12th game, sealing his 40th tour-level match win of a season in which he served a short doping ban.
"I feel like I started off very well in the first set. We both played well in the second set. I had some break chances at 4-3, I couldn't use them," Sinner said.
"He's someone whose peak is high, which I knew before the match. When he was serving for the set he made a couple unforced errors which helped me to come back and win in two."
In another retirement fourth seed Lorenzo Musetti, Sinner's compatriot, was forced to quit against Learner Tien with the American teenager 4-6 6-3 3-0 ahead.
The Italian appeared to be hampered by a thigh or groin issue.
Tien will now play eighth seed Daniil Medvedev who beat second seed Alexander Zverev 6-3 6-3, his fifth successive defeat of the German and eighth in their last ten meetings.
Former US Open champion Medvedev, who has slipped to 18th in the rankings, broke Zverev in the fourth game of the first set and the seventh of the second set.
World No.3 Zverev slammed his racket twice in frustration and ended the match with a final forehand error.
