Cameron Norrie of England lost the quarter-finals 6-4 6-3 becoming the second youngest semi-finalist in Indian wells history.
Alcaraz will face Rafael Nadal.
Alcaraz reached the quarter-finals at the US Open last year.
Nadal won his 19th straight game since the start of the year with a 7-6 (7-0) 5-7 6-4 win over Nick Kyrgios.
Norrie's two break ups in the first set didn't work against Alcaraz, who with incredible strength completed her straight win
"I don't feel much different at all, as the reigning champion," said Norrie.
“It's always nice to support that, and know that it's not just a one-off event. I won [the recent ATP tournament at] Delray Beach and then Acapulco, and I feel like I'm playing at the level I want to be. last year."
Nadal was actually chasing a fourth BNP Paribas Open title, but he realized his loss against Alcaraz, who is almost half his age.
"I think he's unstoppable in terms of his career," he said.
“He has all the ingredients. He has passion. He is humble enough to work hard. He reminded me a lot of things I did when I was 17 or 18 years old.
"I think he has passion. He has talent and a great physical component. And I am very happy, even if, of course, he will be a great rival."