In 2004 his career took a step forward, playing all 25 games and enjoying good form whilst also playing a major role in the club's 2004 premiership win. In 2005 he repeated his efforts, having another excellent year by averaging 16 disposals a game and gathering 57 more possessions than the year before. He also polled third in the club's best and fairest award.
In 2006 Cassisi missed seven games due to injury, but found just enough form in his limited game time to uphold his reputation as one of the most important players at the club. However, at season's end he lost his spot as vice captainn.
After another excellent season in 2007, Cassisi accepted a three year deal with the club, ensuring that he would stay with Port until 2010. He also was reinstated as Vice Captain. In 2008 he undoubtedly played his best season, averaging 22.7 possessions and 6.3 tackles per game and finishing second in the club best and fairest (career best). He also found himself being handed the club captaincy from Warren Tredrea, who stepped down.