Nastase shocked onlookers Friday when he openly speculated about the skin color of Williams' unborn child as Romanian No. 1 Simona Halep was answering a reporter's question about the 23-time grand slam winner.
"Let's see what color it has," Romanian and British reporters quoted Nastase as saying. "Chocolate with milk?"
Williams, who is engaged to Reddit co-founder Alexis Ohanian, announced her pregnancy last week. Ohanian is white.
World No. 1 Williams addressed the controversy via Instagram Monday, adopting a tone of defiance and saying "I am not afraid unlike you."
"It disappoints me to know we live in a society in which people like Ilie Nastase can make such racist comments towards myself and unborn child, and sexist comments against my peers," the post says.
"I have said it once and I'll say it again, this world has come so far but yet we have so much further to go. Yes, we have broken down so many barriers -- however there are a plethora more to go.
"This or anything else will not stop me from pouring love, light and positivity into everything that I do. I will continue to take a lead and stand up for what's right."
Nastase may have retired from tennis 32 years ago, but he appears to be still living up to his old nickname: "Nasty."
The 70-year-old, a two-time grand slam winner and former world No. 1, was provisionally suspended by the ITF from all its events for his behavior at the Fed Cup match against Great Britain.
He hurled abuse at the chair umpire and the opposition during the World Group II playoff clash in the Black Sea resort of Constanta.
Nastase had been thrown out of the tie Saturday after he launched a foul-mouthed rant at the umpire, GB team captain Anne Keothavong and British player Jo Konta after both complained about crowd noise.
Konta, a top-10 player, was so upset by the abuse, her match against Sorana Cirstea was temporarily suspended.
"This is unacceptable behavior by a Fed Cup captain," ITF President David Haggerty said in a statement over the weekend.
Keothavong told reporters: "We expected a patriotic crowd for the Romanian team but we don't expect abusive language to be used.
"What he said directed to both Johanna and myself is ... language that is not appropriate for anyone to speak to any other human."
Another awkward moment came when Nastase put his arm around Keothavong when both captains posed for pictures after the draw presentation, asking for her hotel room number. Keothavong is pregnant with her second child.
Although Konta ended up winning her match against Cirstea Saturday, Britain eventually lost the tie 3-2.
'I don't regret it'
On Sunday, Nastase entered the VIP lounge of the Fed Cup venue before being escorted out for a second time, according to the Press Association.
Later, the ITF said its internal adjudication panel had issued Nastase with "a provisional suspension under the Fed Cup regulations for a breach of the Fed Cup Welfare Policy."
It added: "Under the terms of the provisional suspension, Nastase may not participate in the Fed Cup in any capacity with immediate effect and shall be denied access to, and accreditation for, any ITF event including Fed Cup."
The ITF said it wouldn't make any further comment while the investigation was ongoing.
The suspension means Nastase won't be able to visit any of the four tennis majors as well as Davis Cup and Fed Cup ties.
A former French Open champion, he has been a regular visitor to Roland Garros and Wimbledon in previous years.
George Cosac, president of the Romanian Tennis Federation, wasn't immediately available for comment when contacted by CNN Monday.
In an interview
with British newspaper The Daily Mirror published Sunday, Nastase refused to apologize to the British team.
He said: "I don't regret it and they can send me to prison if they want -- I don't care."
Nastase, a winner of the 1972 US Open and the 1973 French Open, was a controversial figure during his playing days.
"No player in history has been more gifted or mystifying than the 'Bucharest Buffoon,' Ilie Nastase, noted both for his sorcery with the racket and his bizarre, even objectionable behavior," the late American tennis writer and broadcaster Bud Collins wrote on the website of the ATP World Tour.
"He was an entertainer second to none, amusing spectators with his antics and mimicry, also infuriating them with gaucheries and walkouts."