Since 1997, Allegro participated in professional tournaments of ATP. His greatest successes came as a doubles player with three titles and seven other finals. In June 2003 he won along with Roger Federer, the ATP tournament in Vienna. In January 2005 he won together with the Germans Michael Kohlmann the title in Auckland, the same year he won with Roger Federer the grass tournament in Halle. In August 2004 Allegro took along with Roger Federer at the doubles tournament of the Summer Olympic Games in Athens in part. However, the two were knocked out in the second round.
Allegro earned a prize money of U.S. $ 768'327. His best results in the world rankings, he reached into the double rating in October 2004 with number 32 and single in June 2003, was trained with space 210 of the right-handed Allegro by Ivo Werner. Allegro was also have several years regular member of the Swiss Davis Cup team.