The 23-time grand slam champion announced the news at her pre-tournament press conference on Sunday without giving the reasons behind her decision.
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"Yeah, I'm actually not on the Olympic list, so... Not that I'm aware of. If so, then I shouldn't be on it," Williams told reporters on the eve of Wimbledon.
The American, who will turn 40 in September, won singles gold at the London Olympics in 2012 and has also won three gold medals in doubles with sister Venus — in Sydney (2000), Beijing (2008) and London.
At the 2016 Games in Rio, Williams lost in the third round of singles to Elina Svitolina and the siblings were beaten in the first round of doubles.
Before that, they had been 15-0 as an Olympic doubles team.
"There's a lot of reasons that I made my Olympic decision," Williams said.
"I don't really want to - I don't feel like going into them today. Maybe another day. Sorry."
Asked what it will be like to sit out the Summer Games, Williams replied: "I have not thought about it. 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."
Family heartache behind Serena Williams' decision
In May, Williams said she was unlikely to participate in the Tokyo Olympics if she wasn't able to bring daughter Alexis Olympia with her.
"I haven't spent 24 hours without her so that kind of answers the question itself," said the American.
"I haven't really thought much about Tokyo, because it was supposed to be last year and now it's this year.
"And then there is this pandemic and there is so much to think about.
"Then there are the grand slams. It's just a lot. So I have really been taking it one day at a time to a fault, and I definitely need to figure out my next moves."
With overseas fans and athletes' families now banned, it appears as though Williams' decision was made for her.
Other top tennis players such as Rafael Nadal and Dominic Thiem also have said they'll skip the trip to Japan, where the Olympics open on July 23, a year after being postponed because of the coronavirus pandemic.
Roger Federer said he hasn't decided whether to participate in the Tokyo Games and will figure that out after he sees how things go 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.
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