He was awarded his first title at age 18, when he won the WPKL Junior World Super Welterweight Championship during an event near Vienna, Austria on November 15, 1997.

A year later, again in Vienna, Fadi challenged Mohammed Ouali for the vacant WKA Thaiboxing World Middleweight title.

In January 2004, Merza challenged Kamal El Amrani for his IKBO World Middleweight Championship belt.

In front of a sold-out crowd, again going to distance in a spectacular fight, Fadi lost a close decision against his well-known Moroccan adversary.

Fadi Merza claimed the number three spot in the official SuperLeague middleweight rankings with victories over Peter Crook, Jose Reis, Petr Polak and Malaipet Sasiprapa and losing only his fights against Ole Laursen, Sahin Yakut and John Wayne Parr.

Merza was able to return to the winner's column in impressive fashion by knocking out Ümüt Demirörüs and Rico Recineu, in one night to win the King’s Cup qualifier tournament in Switzerland in September 2009.

Fadi earned himself a spot in the King's Cup, the most prestigious and biggest Muay Thai tournament in the world, which is held once a year in Bangkok, Thailand.

Cheered on by over 150,000 spectators, Merza defeated his quarter final opponent, Enriko Kehl from Germany.

Unfortunately, Fadi had to withdraw from the tournament due to a ruptured ear drum, as well as a cut over his right eye, forcing the ring-side doctor to not give his consent for another fight.

Merza was given the chance to fight for a world title again on February 7, 2010, taking on Luca D’Isanto for the ISKA Oriental Light Middleweight Championship.

Staged at the Wiener Stadium in the centre of Vienna, Merza hurt Di Santo in the first round with a knee to the head.

In his sixth defense, Fadi was challenged by Jason Woodham, a WMC MAD champion from England.