Jean-Baptiste began his professional career with AC Ajaccio during the 2006–07 season, making his debut under coach Ruud Krol. He made 25 appearances in his debut season in Ligue 2, and was also an important player during Gernot Rohr's tenure at the club. He also captained the club during the 2010–11 season in which he helped Ajaccio gain promotion to Ligue 1. In his first season in the top flight, Pierazzi appeared in 34 league matches and helped the club maintain its Ligue 1 status. During his time with Ajaccio, Pierazzi appeared in 112 Ligue 2 matches and 67 Ligue 1 matches.