Australian tennis coach Nick Philippoussis faces life in prison in the US after being accused of sexually abusing two nine-year-old girls for almost a year.

Philippoussis snr, 68, who was supported during his first court appearance in the San Diego courthouse by his son, former professional tennis star Mark Philippoussis, pleaded not guilty to 14 counts relating to child molestation on Thursday.

He faces a maximum jail sentence of 21 years to life.

“Shock is more like it,” Philippoussis snr’s lawyer Ryan Tegnelia said, when asked by a reporter if Mark was in disbelief of the allegations.

US prosecutors allege Philippoussis snr sexually abused the two girls he coached in the San Diego area for almost a year and committed the assaults in his car, home and at the California suburb of Fairbanks.

His bail was raised from $US2.5 million to $US9.2 million [$A3.1 million to $A11.5 million] at his arraignment.

San Diego Sheriff Bill Gore said on Wednesday police were looking for “more victims” in the neighbourhoods north of San Diego where Philippoussis snr worked as a tennis coach.

Nick Philippoussis pleaded not guilty to 14 charges relating to child molestation. Photo: AAP

“[Nikolaos] Philippoussis works as a personal tennis coach … and the alleged molestation victims took tennis lessons from him,” Lieutenant Greg Rylaarsdam said.

“In this particular case we are in the infancy of the investigation,” Lt Rylaarsdam said.

“On a lot of cases we go all the way to the end and get all of our facts in order and all of the evidence lined up and then we go out and arrest somebody.

“But in a case like this where there is the potential of ongoing victimisation we developed probable cause and went out and arrested him when we had that. Our investigation is ongoing from today.

“We have done an investigation up until now which leads us to believe that this did in fact occur and we arrested him and essentially took him out of the population.”

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.

He said Philippoussis snr’s family, including Mark, were supporting him and understood “the serious nature of these charges”.

“This caught everyone completely flat-footed,” Mr Tegnelia said.

There are no apparent links to victims in Australia.

– with AAP