Venus and Serena Williams' father Richard has suffered irreparable "brain damage" and "dementia" following a stroke, his doctor has revealed in court documents.
The court papers also show the two sisters have become embroiled in a court battle against their stepmother.
Their 77-year-old father and former tennis coach is "totally incapacitated to conduct any business", according to his doctor.
He has been fighting a divorce battle with ex-wife Lakeisha Williams, 40.
In 2018, the Daily Mail reported he had suffered two strokes which left him barely able to speak.
He has been living far from the public eye since then.
Richard Williams claims his ex-wife forged his signature to change the mortgage deed on his Florida house to her name.
He also claims an associate of his ex has posed as him to allow her to borrow nearly $300,000 of equity on the house, to start a trucking business.
The associate, David Simon, has been fighting to get the case thrown out of court and denies any wrongdoing.
Simon is trying to get sisters Serena and Venus involved. He's asked their dad to "describe all income, real property, personal property, or gifts received from Venus or Serena Williams since 2015 until the present".
Their dad has refused and argues that has nothing to do with the case.
Richard Williams gave an incoherent deposition in June 2018 and his doctor says the stroke has affected his "brain function", The Sun reports.
"As you are well aware, you suffered damage to your brain tissue during a stroke," his doctor said.
"The complications after the stroke included inability to remember things which happened recently and incidents that happened years ago.
"You will remember, immediately after the stroke, your muscles weakened and slowly you were able to regain strength. You also have problems speaking and finding the right word to use.
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"You have recovered from problems forming your words. However, you have not recovered from what is called dementia," the court document states.
"Your ability to do household business transactions such as writing checks, paying bills, and maintaining records is lost. You still at times find it difficult to use the right words. This is all part of cerebral vascular disease and stroke."
Simon has argued in court that Williams' testimony should not be taken into account.
In his abandoned first deposition, Williams didn't know whether his lawyer was a lawyer or family member, saying "I'm not sure".
"That's pretty serious, because almost everybody that you would speak to knows who their relatives are," the cross-examining lawyer told him.