He began to swim at the age of 8, becoming a member of the Italian national team in 1989. His first important medal was at the European LC Championships 1993 in Sheffield, where he won a bronze medal in his favourite race, the 200 m backstroke. He repeated the result in the European LC Championships 1995, this time as a member of the 4×200 m freestyle relay.
Merisi presented at the 1996 Summer Olympics with the best time of the year in the 200 m backstroke, 1:57.70. This is still today the Italian record. In the race, however, he was not able to repeat this outstanding result: he gained only a bronze medal after Americans Brad Bridgewater and Tripp Schwenk, both with times largely superior to that time.
A talented and long-career swimmer, captain of Italian team in his last years, Merisi took part to two more editions of Summer Olympics, including 2000, in which he was ranked again amongst the favourites in 200 m backstroke and arrived 5th. His results include two more silver (1997 and 2000) and a bronze medal (1999) at the European Championships, all in the 200 m backstroke.