Berrettini was blown away by Nadal in the opening two sets, before launching a spirited comeback that ultimately fell short.
The commentators covering the match were left confused over why the usually-explosive No.7 seed had been so muted to begin the contest.
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Berrettini even received a visit from one of the medical staff on hand during the opening set, but did not receive any visible treatment.
“You get to the semifinals, the end stages of the tournaments, normally you’re a little bit tired,” he said after the match.
“I felt like I was a little bit more (tired) because I have been struggling some days with my stomach, and then I twisted my ankle before the tournament and during the tournament.
“So there were some things that I had to handle that are normal to handle. But you’re always wishing or trying to not make that happen.”
The 25-year-old did not blame his loss on the stomach issues, admitting Nadal deserved the victory, but said he was proud of his response after the initial hiccup.
“The first two sets weren’t what I expected, what I wanted from myself,” he said.
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“I was, I wouldn’t say flat, but I wasn’t in the right mood, in the right mental mindset and for so many reasons.
“I think for the first set time I’m in the semi-finals, against Rafa, so many … there can be so many reasons.
“But the fact is that I was just like that, and it was good that I had a really big and good reaction. This is the thing that I’m most proud and happy about.”
Before Berrettini flipped the script with a 6-3 third-set win, he was shredded by two-time Australian Open winner Jim Courier for his approach to the match.
“You’re not going to beat Nadal if you let him control play, it doesn’t happen,” Courier said on Nine’s coverage of the semi-final.
“Nadal doesn’t beat himself, he’s too consistent and has too much margin for error.
“Berrettini so far has not approached this as a shootout, and that’s what this needs to be. Kill or be killed on every single point.
“Berrettini right now has been far too conservative and I don’t quite understand it. That’s basically a death sentence.”
Both men played indoors during the match, with the Rod Laver Arena roof closed due to wild weather during the afternoon. Berrettini admitted he took too long to adjust to playing indoors.
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“It was the first time that I played with the roof. It was different conditions. I struggled,” he said.
“At the beginning I couldn’t really find the rhythm, and the conditions were a little bit different. It took me a while to adapt, but at the same time Rafa was playing really good.
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“So I learned that you have to be ready any time, and I’m learning this every day since I started playing tennis. It’s a sport that you have to be able to adapt to everything that is happening. Today he was just way better than me.”
Berrettini leaves Melbourne Park having achieved a career-best finish, with last year’s fourth-round exit his previous best.