Sinner staves off Chinese wildcard to set up Alcaraz final in Beijing
The world number one and defending champion defeated the 96th-ranked Chinese wildcard Bu Yunchaokete 6-3, 7-6(3) to avoid a major upset.
The Italian faces Spanish second seed Alcaraz, who was in fine form in a 7-5, 6-3 victory over Russian third seed Daniil Medvedev in the other semi-final.
In front of a packed house on China's national day, US Open champion Sinner was facing a player he had never met before and who was competing in the biggest match of his life.
China's 22-year-old Bu was also in the form of his life, having similarly reached the semi-finals last week in Hangzhou.
Bu had Sinner under pressure on the Italian's serve early in the first set, but Sinner staved off the threat and the storm gradually blew itself out as the set wore on.
They went to the tie break in the second set, with Sinner serving an ace to convert his third match point after just over two hours.
Nobody has won more matches on tour this season than Sinner, 23, who failed two doping tests in March but was cleared of wrongdoing and allowed to keep playing.
Tennis authorities accepted Sinner's explanation that trace amounts of a banned steroid unintentionally entered his system from a physiotherapist who used a spray containing it to treat a cut, then provided massage and sports therapy.
The World Anti-Doping Agency (WADA) reignited the case on Saturday, saying it had appealed the decision and is seeking a ban of up to two years.
Sinner has admitted to having sleepless nights over the issue.
Four-time Grand Slam champion Alcaraz reached his first final in the Chinese capital and is in ominous form, yet to drop a set.
"I think I played really well. Obviously if I want to beat Daniil, I have to play really good tennis, high-quality tennis," said Alcaraz, who is chasing a 16th ATP Tour title - which would equal Sinner.
"I felt great on the court once again, so I'm really happy about it. I think I couldn't ask for a better semi-final."
The final is on Wednesday.
Shades of Mbappe
In the women's draw, 595th-ranked home hope Zhang Shuai - one of the storylines of the tournament - rolled into the quarter-finals to make more history.
Her 6-4, 6-2 victory over Magdalena Frech was her fourth win in Beijing and all in straight sets - having come into the event after losing 24 singles matches in a row.
The barren run lasting more than 600 days was the second-longest on the WTA Tour in the Open Era, which began in 1968.
The 35-year-old Zhang, a former two-time Grand Slam quarter-finalist who has been plagued by injury, said she had come close to quitting singles tennis before coming to Beijing.
"I had no idea how to win and I didn't want to lose another, especially with all the Chinese fans," she said.
"Walking back to the locker room, I didn't know if I should continue or not before this tournament."
The former top-25 player will face Spain's 19th-ranked Paula Badosa in the last eight.
Zhang is the lowest-ranked player to reach this stage of the China Open and struck a defiant pose - a tribute to Kylian Mbappe - after beating 31st-ranked Frech.
Former world number two Badosa dumped out US Open finalist Jessica Pegula 6-4, 6-0 to make the second-seeded American the biggest casualty so far.