The 20-year old midfielder has extended his stay with the Sky Blues until the end of the 2016/17 Hyundai A-League season in what is a major coup for the club.
Antonis, who has made over 50 appearances since his debut in 2010/11, admitted his delight at committing himself to the Sydney cause after being given an opportunity to shine at the Allianz Stadium.
"I'm thrilled to be extending my time with Sydney FC," Antonis said.
"I'm glad to be able to concentrate on playing now and I'm keen to develop even further under Arnie and the team.
"There was never any hesitation when the offer came through and I'm confident I can achieve my goal of winning trophies here."
Antonis has really shone in the A-League over the past few seasons which has been evidenced by recent call-ups to the Australian national team.
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