Alcaraz arrived in Miami in top form after winning the Indian Wells trophy, but his hopes of becoming the first man to complete the Sunshine Double since Roger Federer in 2017 were dashed in a 6-2, 6-4 defeat.
It was the Spaniard's second consecutive defeat by Dimitrov following last year's three-sets loss in the Shanghai Masters.
"He played amazing tennis, almost perfect. I couldn't find solutions. I couldn't find a way to make him feel uncomfortable on the court. It was a great game from his side," Alcaraz said.
"I left the court thinking that probably I had to change my game a little bit earlier, but anyway, I feel like I played good tennis. Not perfect but good.
"But I have a lot of frustrations right now, because he made me feel like I'm 13 years old. It was crazy. I was talking to my team saying that I don't know what I have to do. I don't know his weakness. I don't know anything."
Twice Grand Slam champion Alcaraz said that he was already looking ahead to his next meeting with Dimitrov and planned to switch his game to extend his 3-2 win-loss record over him.
"Probably the next match that I'm going to play against him, and I'm sure that there's going to be another one, I'll try to do a lot of different things," Alcaraz added.
"Let's see how it's going to be."
Dimitrov, ranked world number 12, will play his fourth semi-final of the season when he takes on Alexander Zverev and will enter the top 10 if he wins.
"I'm not going to think from now what I can do. It's just very important to stay in the moment," Dimitrov said.
"That's what our game is about. That's the beauty of our game and it's always the most simple things that are the most difficult to do.
"This is actually what I did tonight. I was just trying to do everything as simple as possible ... I think in a way, same thing tomorrow, the same thing every match I'm going to be playing from now on. There's no pulling back."
