“I’m actually not on the Olympic list, so… Not that I’m aware of. If so, then I shouldn’t be on it,” she said.
She refused to divulge the reasons for her decision.
However, a ban on foreign fans as well as family members at the Olympics would have meant Williams being separated from her daughter Alexis Olympia.
The Olympics have already been delayed by a year and organisers have been forced to introduce strict measures to fight Covid-19.
“There’s a lot of reasons that I made my Olympic decision,” she said.
“I don’t feel like going into them today. Maybe another day. Sorry.”
Williams is the joint most successful Olympian in tennis history in terms of gold medals alongside sister Venus having won one gold medal each in singles and three in doubles.
Venus edges the medal count slightly as she also won a silver in mixed doubles in 2016 in Rio.
Serena, who is bidding to equal Margaret Court’s record of 24 Grand Slam singles titles at Wimbledon, added she had not really thought about what it would mean to miss the Games.
She won the 2012 Olympic singles title in London on the same Centre Court where she has won seven Wimbledon singles titles.
She also won doubles gold with sister Venus in Sydney (2000), Beijing (2008) and London.
“I have not thought about it (missing the Games),” said Williams, who plays world number 100 Aliaksandra Sasnovich of Belarus in the first round at Wimbledon.
“In the past it’s been a wonderful place for me. I really haven’t thought about it, so I’m going to keep not thinking about it.”
Nadal announced earlier this month he would be missing Wimbledon and the Olympics so as to look after his body and prolong his career.
His great rivals Roger Federer and Novak Djokovic have yet to decide whether they will go to Tokyo.
Federer, who like Serena is 39, said Saturday it will depend on how he fares at Wimbledon.
Two-time defending Olympic singles champion Andy Murray will be going, fitness permitting, as he has been selected in the Team GB squad.
Austrian Dominic Thiem will be another absentee.
The 27-year-old 2020 US Open champion announced his decision on the same day as Nadal and just before he withdrew from Wimbledon due to a wrist injury.
