Serena Williams’ father Richard is suffering from irreparable "brain damage" and "dementia", his doctor has revealed in public available court documents.

The court papers also reveal tennis ace and her sister Venus could become embroiled in a court battle with their stepmother.

The 77-year-old is "totally incapacitated to conduct any business", the documents, which include testimony from his own doctor, state.

The former tennis coach has barely been seen in public over the last few years, as he’s been fighting a messy divorce battle with Lakeisha Williams, 40.

He suffered two strokes which left him barely able to speak, Daily Mail reported in 2018.

Richard claims in court documents that Lakeisha forged his signature to change the mortgage deed on his house in Palm Beach Gardens, Florida, into her name.

He then claims an associate of Lakeisha’s pretended to be him, so that she could borrow nearly $300,000 of equity on the house from a lender named David Simon to start up a trucking business.

Richard is suing both Lakeisha and David, who have denied wrongdoing have been attempting for over a year to get the case thrown out of court.

Now David is trying to involve tennis stars Serena and Venus and has asked their dad to "describe all income, real property, personal property, or gifts received from Venus or Serena Williams since 2015 until the present".

Richard has refused to give an answer, arguing through his lawyer Thomas Gruseck that it has nothing to do with this case.