The father of former tennis star Mark Philippoussis has denied sexually assaulting two nine-year-old girls while he was a private tennis instructor in California.

Nikolaos Philippoussis, 68, pleaded not guilty to 14 counts of committing lewd acts and sex acts with children at a hearing in Vista.

He could face life in prison if convicted.

Nikolaos PhilippoussisNikolaos Philippoussis (Hayne Palmour IV/AP)

Philippoussis is accused of molesting two of his tennis students for more than a year, beginning in June 2016.

Prosecutors say the encounters took place at his home, in his car and at a San Diego County country club.

Philippoussis was arrested on Tuesday at his San Diego home.

Nikolaos Philippoussis(Hayne Palmour IV/AP)

At the hearing, his bail was raised from 2.5 million to 9.2 million dollars (£7m).

“It is quite a colossal amount,” defence lawyer Ryan Tegnelia said.

Philippoussis, an Australian citizen, was ordered to surrender his passport if he does post bail, but his lawyer told the San Diego Union-Tribune that was impossible.

Mark Philippoussis Mark Philippoussis (Jon Buckle/EMPICS)

Mark Philippoussis, 40, a two-time Grand Slam finalist who was once ranked eighth in the world and helped Australia win the Davis Cup in 1999 and 2003, was in court for the hearing but declined to comment, the newspaper said.

His father coached him for much of his career.