Philippoussis, 68, looked frail as he sat in handcuffs, leg irons and a blue prison jumpsuit in a closed off area of the courtroom just north of San Diego on Thursday.

At one point he peered through a small opening, saw his son, the former professional tennis star Mark Philippoussis, at the back of the public gallery, and smiled and waved.

Philippoussis snr entered not guilty pleas to 14 charges relating to the assaults of the two girls who he coached.

He faces a maximum jail sentence of 210 years to life if convicted. The charges include oral copulation and lewd acts on the girls.

Deputy district attorney Garret Wong described what Philippoussis snr allegedly did to the girls as "a parents' worst nightmare".

Philippoussis Snr's lawyer Ryan Tegnelia however said his client was innocent and would fight the charges.

Philippoussis' family, including Mark, were in shock after the arrest but were supporting him.

"This caught everyone completely flat footed," Mr Tegnelia said.

Philippoussis snr will likely remain in jail in San Diego until the end of the court proceedings as Judge David Danielsen agreed to substantially raise his bail from $US2.5 million to $US9.2 million.

Prosecutors do not expect Philippoussis snr to make the bail.

Mr Tegnelia said Mark Philippoussis has offered to provide any comfort for his father during his jail stay.

Because of the child sexual abuse charges, Philippoussis snr, who has been in San Diego's downtown jail since Tuesday's arrest, is being kept in protective custody.

AAP