But she also had an unlikely weapon in her arsenal - a sleek, black catsuit.
With the Lycra unitard hugging her commanding frame, Williams possessed an aura of almost mythic proportions. She was a Glamazon, a potent cocktail of Wonder Woman and Xena the Warrior Princess with a touch of Buffy the Vampire Slayer. She was daunting, but she also was sexy - in that whips-and-handcuffs kind of way.
Above all, Williams conveyed one message to her opponents: "Mess with me and die."
Power dressing, it seems, has crept onto the court.
"She was a goddess," said fashion observer David Wolfe, creative director at Doneger Group, a retail consulting company in New York. "She was wearing what the powerful heroines in comic strips and action movies wear. It was like The Avengers, but on the tennis court.
"It's intimidating to the competition," he added. "Fashion can make a power statement."
In tennis, flashy fashions have hit the headlines periodically over the decades. Women's French Open champ Suzanne Lenglen raised eyebrows in the 1920s when she wore a dress that exposed her ankles. And Gussy Moran provided rousing fodder for the press when she wore lace undies on court for the first time in the 1940s.
But in recent years, style statements on the court and in other women's sports have become more frequent. Williams' sister, Venus, caused a stir early this year when she donned a tight top with spaghetti straps at the Australian Open. High jumper Amy Acuff has been known to compete in fur-trimmed or sequined ensembles. And Nancy Kerrigan began the wave of couture skating when she took to the ice at the 1994 Olympics in a sparkly Vera Wang piece.
Susan Casey, managing editor of Sports Illustrated Women, said female athletes have started expressing their individuality partly because it makes them more confident.
"We're more style-conscious and media-conscious now," she said. "I'm a swimmer, and when I'm swimming in a race, I like to wear an unusual bathing suit. When I swim, I'm expressing myself. I feel very secure. I know when I'm swimming that I rule my world.
"Take Missy Giove," Casey added, referring to the mountain biker who was two-time U.S. downhill champion. "She used to wear a dead piranha around her neck. It just shows people that you are crazy and you mean business."
Ken DeHart, a United States Tennis Association master professional who's been teaching the sport for 30 years, said he offers a class at the San Jose Swim and Racquetball Club called "Defeating the Monsters of the Mind," in which he discusses selecting a match wardrobe that makes you feel powerful.
"Part of playing in the mental zone is finding out what gets you there," he said. "And dressing is an important part of the ritual. When you go on a date, you'd dress in a way that makes you feel confident. The same way, when you go out to play, part of being successful is choosing the clothes that make you feel confident."
Creative director Wolfe had another theory for fashion statements like Williams'.
"I think many women regard fashion as a competition," he said. "So it's natural that women who are in competitive sports would have the same competitive urge when it comes to presenting themselves. Whether it's women or men sitting in an office waiting for a job interview, they certainly scope each other out. If you can take that competitive edge and express it with what you wear, why not give yourself that edge? It's all about winning."
In the case of Serena Williams, an interest in fashion design sparked her experimentation with her court wardrobe. Williams, who has taken a fashion course at the Art Institute of Florida, has been designing for Puma and professes to love her new $65 catsuit, which is now available in stores. Williams accessorized her catsuit on the court with a $29,000, 9-carat Harry Winston bracelet.
"This is an innovative outfit," she said at the U.S. Open. "It's a new time, I mean, like I said, it's really nice. It's really sexy. It just kind of clings. It's really cute."
Besides comfort and confidence, there are added perks when an athlete like Williams makes international headlines because of her outfit. The more media attention, the likelier she is to get more endorsements.
Still, it helps when athletes can back up their looks with solid skill. Witness Anna Kournikova, who has attracted endorsements more for her pouty Playboy mien than her tennis skills. She was booed off the court this week.
That's even true on the ice, where costumes are an integral part of the competition. "In ice skating and ice dancing, the costumes have become more and more a part of the performance," said Barbara Kerin, an assistant professor of fashion design at the Fashion Institute of Technology who specializes in sportswear. "They convey an image that is in sync with the music and the mood and the atmosphere and emotion that the skater is trying to convey. But they work in addition to the fact that she can do a triple Axel."
And with Williams' combination of power-dressing and skill, the effect this week was lethal. As Williams' first opponent, Corina Morariu, conceded shortly after being vanquished from the tournament, "She looks great. I mean, geez. You know, I couldn't pull that off."
Sun staff sportswriter Sandra McKee contributed to this article.