Los Angeles (AFP) - The father of retired Australian tennis star Mark Philippoussis pleaded "not guilty" on Thursday to charges he sexually assaulted two nine-year-old girls he coached in California.

Nikolaos Salvator Philippoussis, 68, is being held on US$9.2 million (S$12.5 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.

According to a criminal complaint, the alleged crimes took place over the last year at the Fairbanks Ranch Country Club, in a car and at the coach's residence.

He faces a maximum of 210 years to life in prison if convicted, American prosecutor Garret Wong said.

A readiness conference in the case was set for Aug 31 and a Superior Court preliminary hearing for Oct 5 in Vista, California.

The San Diego Union Tribune reported that Mark Philippoussis, a Davis Cup champion with Australia and former 2003 Wimbledon finalist, attended Thursday's court proceedings.

Nick Philippoussis' attorney Ryan Tegnelia told the newspaper that his client "maintains his innocence, obviously".

"He's looking forward to diligently defending his case," Tegnelia said.