Berrettini, 23, was competing in his third singles final of the season, following victory in Budapest in April and defeat in Munich last month, on clay on both occasions. The narrow win over Auger-Aliassime marks the Italian's first tournament victory on grass.
Berrettini secured a crucial break of serve early in the first set, creating a deficit that Auger-Aliassime was unable to close. Neither player earned any break points during the remainder of the set, and Berrettini served out the opener 6-4 after 33 minutes of play.
The world No. 30 increased the pressure in the second set, forcing Auger-Aliassime to save five break points. The 18-year-old held his nerve well, however, and took the set to a tiebreak.
Auger-Aliassime was agonisingly close to forcing a final set in the back-and-forth breaker, saving two championship points and even earning five set points of his own. The No. 7 seed failed to convert any of those opportunities though, and fell to a third championship point to bring the one hour, sixteen minute-long set to a close, as well as the match.
Like Berrettini, Auger-Aliassime was contesting his third final of the year, but the Canadian's search for a first career title continues.