Nadal survived 10 aces from Fritz and broke the service three times to outperform the 22-year-old American.
"I couldn't be happier. I played a great event from the beginning to the end," Nadal told reporters.
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Acapulco was the first big title that I won in my career, so to be able to stay here after 15 years is amazing. I can't thank enough the people who make me feel at home every single time.
The world No. 2 Nadal captured the title 15 years after he won his first in 2005. He also won the crown in 2013.
Nadal stormed through the draw without dropping a set and improved to 20-2 in the event overall.
Fritz had reached the final by rallying from a set behind and 2-4 deficit to John Isner but he could mount no such comeback against Nadal.
Fritz is the only American besides Sam Querrey in 2017 to reach the final in Acapulco.
Despite the defeat, Fritz reached his first final at an ATP 500 event.
"He's one of the best players to ever play the game and he showed me why that is tonight," Fritz said.
"This is one of my favourite tournaments to come to every year. I'm just glad that I could have a good week at one of my favourite events. I felt a lot of love all week."
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