Don't believe it, says the tennis superstar.
In an interview with CNN's Rachel Nichols, Williams confessed that she gets just as tense as any other human being.
"You should definitely come to my practices 'cause I'm not relaxed at all," she told Nichols in an interview from Cincinnati, Ohio, where Williams was playing in the Western & Southern Open.
"In Australia I was, like, puking before I walked on the court. At the French (Open) I just -- it was only by the grace of God and miracle that I won that."
But, she added, she's learned how to handle the jitters.
"I've learned that I have a tough mind and I've faced a lot of adversity," she said.
Her fierceness has not gone unnoticed.
"What has made Serena the best on the women's side of tennis, possibly in all of the sport, is not only her skill, but her determination to win," wrote Isportsweb.com's Rachel Grim
'I get chill bumps'
Williams, 33, is preparing to become the first woman since Steffi Graf to pull off a calendar year Grand Slam, which encompasses victories in women's singles at the four majors: the Australian Open, French Open, Wimbledon and the U.S. Open. Besides Graf, only two other women have accomplished the feat: Maureen Connolly in 1953 and Margaret Court in 1970.
Williams has already achieved a "Serena Slam" as the current title holder of the most recent four majors, going back to last year's U.S. Open. It's her second "Serena Slam" -- she also had four straight major wins in 2002-03.
Another U.S. Open title would give her five major victories in a row, a record exceeded only by Connolly, Court and Martina Navratilova, all of whom won six straight majors. (Navratilova won three in 1983 and another three in 1984, barely missing the Slam both years.)
Williams' victories have been among the highlights of a spectacular year for female athletes, a year that's included a World Cup title for the U.S soccer team
and the addition of women as coaches in the NBA and the NFL
She hopes the confluence is inspiring for young athletes.
"You know, I get chill bumps. Like, literally thinking about all this stuff that, you know, the women are doing in sports," she said.
"I think that we as women are on the rise," she said. "There is a lot of stuff outside of sports, a lot of women CEOs that I look up to and a lot of women that are really empowering and that are doing really well. And it is something that is just a great thing."
'I don't like to sit still'
Williams has been "on the rise" -- or on top -- since first picking up a tennis racket as a young child.
Initially in the shadow of her older sister, Venus -- who won four major titles in 2000-01 -- Serena Williams quickly came to dominate the sport, becoming the world's No. 1 player in 2002. Overall, she's won 36 major titles, including 13 doubles wins and two mixed doubles trophies.
What's more, she appears as strong at 33 -- an age when many players are on the downside of their careers -- as she was at 21.
With age has come more contemplation, if no loss of fire. Williams has an active Twitter account and has tweeted comments about social issues -- as well as images of her workouts -- to her 5 million followers. In one she referenced Christian Taylor, an unarmed black football player who was fatally shot by police
"I think a lot of people in America and outside of America are kind of frustrated and concerned and really scared," she said. "I really think it just boils down to people as a nation pulling together. And it's not just me. And it's not just me speaking out. There's a lot of people that are speaking out. It's a whole nation that's coming together. And we're just asking the same question, 'Why?' "
Moreover, she's already working on another career, taking pre-med college classes. She said she was partly inspired by Venus, who has been diagnosed with Sjogren's Syndrome, an immune-system illness.
"I like education and I don't like to sit still. And I don't like any free time," she said. "And I like to keep going and going. And I realize that that's just a craziness that I have. And that's just who I am."
Of course, Williams does have a lighter side, one that's been frequently remarked upon: her love of karaoke
Could a singing career be in the offing as well?
Don't count on it, said Williams.
"I'm not a good singer at all by any means," she said, though she always enjoys karaoke.
No, if she wins the U.S. Open, she has other plans.
"I'm gonna take a vacation," she told Nichols. "I'll just take a deep breath and disappear."
That sounds positively ... relaxing.