No.8 seed Berrettini has enjoyed a superb debut tournament in the Spanish capital, the 25-year-old dropping just one set on his way to becoming the first Italian to make the Madrid Open final.
Berrettini, who won the Serbia Open last month, is just the third Italian ATP Masters 1000 finalist since the series was established in 1990, after Fabio Fognini and Jannik Sinner.
"It's a great, unbelievable feeling, especially coming from Monte Carlo where I wasn't feeling that good," Berrettini said.
"I won in Belgrade and now I'm in the final, so I'm really looking forward to playing against Sascha [Zverev]. I hope I enjoy it tomorrow the way I enjoyed tonight.
"I saw him [Zverev], he's playing really good. He's solid. He moves well. For his height, he's really good. He serves well. I mean, he's in the final. He beat Rafa [Nadal] and Dominic [Thiem], probably the best players on clay.
"It's going to be a challenging match, but I'm in the finals. I guess the best two guys are going to play each other."
Berrettini returned supremely throughout Sunday's (AEST) match, nullifying Ruud's usually potent serve, and believes his aggressive approach was key to victory.
"My serve is my weapon, but today I think I returned a lot and I put pressure on him," Berrettini said. "I was just playing more aggressive. He's kind of like me, he likes to run around the forehand and play with spin. But I guess today I was playing better than him.
"The other time he beat me, so it's always a great fight against him. He reached three semi-finals in a row so he was feeling confident. I guess that's why I'm maybe even more happy with my win today."
In the earlier semi-final, Zverev followed up his famous win over home favourite and 20-time grand slam champion Rafael Nadal in the previous round by defeating Dominic Thiem.
Austrian Thiem had won the past four meetings between the two, including in last year's Australian Open semi-final and the US Open decider, where he came from two sets down to win in an incredible fifth-set tie-break.
However, Zverev was not to be denied this time, a 6-3 6-4 result keeping him on course for a repeat of his 2018 triumph at the event in the Spanish capital.