Nikolaos Salvator Philippoussis, 68 is being held on $9.2 million bail and was ordered to surrender his passport in the event that he posts bail and is released.
He was working as a private tennis instructor in San Diego County at the time of the alleged assaults and is charged with two counts of sexual penetration/oral copulation with a child under 10 and a dozen counts of lewd act on a child under 14.
He faces a maximum of 210 years to life in prison if convicted, prosecutor Garret Wong said.
A readiness conference in the case was set for August 31 and a preliminary hearing for October 5 in Vista, California, Superior Court.
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The San Diego Union Tribune reported that Mark Philippoussis, a Davis Cup champion with Australia and former Wimbledon finalist, attended Thursday's court proceedings.
Nick Philippoussis's attorney Ryan Tegnelia told the newspaper that his client "maintains his innocence, obviously.