Berrettini, the second seed, had three break chances in the second set, and Ruud fought them all off. The set then was decided by a tiebreaker, and Berrettini couldn't capitalize on his chances, dropping points on his serve four times - including the deciding point.
An energised Ruud then powered through the final set to win the ATP 250 event in two hours, 34 minutes. It was the ninth career tour title for the Norwegian - his eighth on clay.
Ruud now is 16-0 in Switzerland. He also has won the Gonet Geneva Open twice.
Ruud, 23, moved to 3-2 all-time against Berrettini, 26, with the win.
Hamburg European Open
Italian Lorenzo Musetti (20) squandered five championship points in the second set but came back to stun top seed Carlos Alcaraz (19) and win his first ATP Tour title 6-4, 6-7 (6), 6-4 in Hamburg, Germany.
It was the first ATP final for the 20-year-old Musetti, who managed to best one of the stars of the 2022 season.
Alcaraz, who will rise to a career-high number five in the rankings on Monday, has four tour wins this season.
Musetti had two championship points slip away while serving for the match at 5-4 in the second set, then lost three more in the tiebreaker.
This was the first tour meeting between the Spaniard and Musetti, who will reach a career-high No. 31 in the rankings on Monday.
Alcaraz is now 14-1 at ATP 500 events this year.
