Tennis instructor Nikolaos "Nick" Philippoussis has pleaded not guilty to 14 counts of sexually assaulting two nine-year-old girls he was privately coaching and faces a 210-year sentence, if found guilty.

The 68-year-old Australian appeared in handcuffs at the San Diego Superior Court, where he was slapped with a massively increased bail of $US9.2 million ($NZ12.3 million), up from the previous $US2.5 million ($NZ3.3 million).

His lawyers did not oppose it, saying it was commensurate with such grave charges - and they said he could not raise it anyway.

Philippoussis's son, Mark, 40, the former tennis star, who like his father lives in San Diego, was in court to support his father, but made no comment to the media.

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EXTREMELY SERIOUS

Deputy district attorney Garret Wong said the numerous alleged assaults took place over a year and were regarded as extremely serious.

"I think what's most concerning is the defendant being in a position of trust, and it's probably any parent's worst nightmare of having someone in that position of trust - a teacher, a coach, an instructor - and then to violate that trust repeatedly over this amount of time, yeah, it's serious," he said after the brief arraignment.

The assaults are alleged to have occurred at the Fairbanks Ranch Tennis Club, at Philippoussis's home and his vehicle.

Wong declined to comment on how the complaints came to light or how long the investigation had been running.

But police said separately that they believed Philippoussis was unaware of the investigation, until he was arrested at his home on Tuesday morning.

They said they had put strategies in place to stop him "taking a Qantas flight home" had he become aware they were moving on him.

210 YEARS TO LIFE

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Each of the 14 counts carries a 15-years-to-life sentence range. The most serious are counts one and two, which allege "oral copulation/sexual penetration with a child ten years old or younger".

The remaining 12 counts, all described as a "lewd act upon a child", carry additional special allegations, meaning they were committed against more than one victim.

"Adding that all up, the maximum exposure is 210 years to life," Wong said.

The lawyer for Philippoussis, Ryan Tegnelia, said he was yet to see the evidence that police had gathered against his client.

"He's calm, he maintains his innocence obviously, he's looking forward to diligently defending his case," he said.

"He's anxious to see what evidence they have against him and he's anxious to provide evidence of his own, which he's informed us he believes will assist him in his defence."

Asked about the bail, Tegnelia described it as a "colossal amount", but said it was in line with what is often demanded in such cases.

FAMILY MATTERS

Tegnella said the arrest "took the entire family by surprise".

But he said: "Mark's doing very well. He's of course very concerned for his father and wants to help in any way that he can.

"He understands the serious nature of the charges."

Philippoussis is being held in protective custody in the San Diego Central Jail. He has several minor health issues, but is otherwise considered well.

Tegnelia said Australian consular officials had made some attempts to make contact with him, but had so far been unsuccessful.

The case will be mentioned again in 10 days.