-- Serena Williams' serve has been widely acknowledged as the most dangerous weapon in the history of women's tennis.
Perhaps we should be talking about her mind.
At the age of 32, she is proving the old Clairol hair-coloring slogan to be true: She's not getting older; she's actually getting better.
This defies the career arc of virtually every previous tennis professional who has come before her.
"I don't know," she told reporters after advancing to the fourth round with a 6-3, 6-3 victory over No. 31 seed Daniela Hantuchova. "I feel like, you know, in life, 32 is young. In sports, it's old.
"But for whatever reason, I feel like I just never was really able to reach my full potential. And I feel like recently I just have been able to do a little better. I just keep trying to improve on everything."
On Sunday, she'll play No. 14 seed Ana Ivanovic in a match of two former Grand Slam singles winners that will vault the winner into the quarterfinals.
How have past champions done when they creep into 30-something territory? Not particularly well, especially relative to their glory days.
The obvious comparison is Roger Federer, who is also 32 and also owns 17 Grand Slam singles titles. Since turning 30 after the 2011 Wimbledon tournament, Federer has played 10 majors, winning only one, Wimbledon in 2012. Since turning 30 near the end of the 2011 season, Williams has won four of eight Grand Slams.
The great Steffi Graf played one Grand Slam after her 30th birthday, in 1999 at Wimbledon. She lost in a straight-sets final to Lindsay Davenport and promptly retired. Chris Evert played 16 majors after hitting 30 -- and won two, the French Open in 1985 and 1986. Martina Navratilova played 23 majors and won three Grand Slam singles titles, but two of those came in her 31st year. Margaret Court, who leads all women with 24 major championships, played eight past 30 and won three, all of them coming in her 31st year.
And then there's this: After their 32nd birthdays, Evert, Navratilova and Court won a combined total of only one major -- Navratilova's title at Wimbledon in 1990 at 33. The way Williams is playing, it wouldn't be terribly surprising if she won two or three this year.
Even when she's missing, you can see the wheels turning in her head. Against Hantuchova, she had a string of five consecutive unforced errors in the first three games.
"I was just trying to get my bearings, trying to figure out [how] to do the right thing," she said. "I wasn't fully on today, so I was just trying to see if I could just go for it and how far out they were going to be, trying to make them."
Like an expert marksman dialing in on a target, she soon got the calibration right -- and annihilated Hantuchova, who later opined on Facebook that Serena is the best women's player ever. Is the same punishment in store for Ivanovic? History says … yes.
The two have met four times -- three, curiously, at the US Open -- and Williams has never lost a set. Ivanovic has never gotten more than four games in a frame.
"Serena is on top of the game for so long now," Ivanovic told reporters after beating Samantha Stosur in three sets. "She's someone we admire, actually. It's going to be a very tough task. But I look forward to that challenge. I've been playing some good tennis. It's going to be nice to see where I'm at."
Ivanovic, 26, is playing well, coming off the 12th title of her career in Auckland, a three-set victory over Venus Williams. She has won eight matches in a row. It's easy to forget that Ivanovic was a finalist in Melbourne six years ago, when she lost to Maria Sharapova. Four months later, she won the only Grand Slam singles title on her résumé, at Roland Garros in 2008. That was part of the sweet spot of her career, when she reached three major finals in a span of five in 2007-08.
Her year-end ranking, which crested at No. 4 in 2007, has consistently hovered between Nos. 13 and 22 for the past five years.
"Ivanovic, I think, is doing really well," Williams said. "She's on a little winning streak herself."
Stosur was asked how she thought the match would go.
"Anytime you go on the court, there's always a chance," the Australian said, not sounding overly optimistic about Ivanovic's chances. "If [Ana] can keep serving well, be dangerous on her forehand, it will certainly give Serena something to think about."
That isn't likely to trouble Serena, an athlete who somehow has convinced herself that she's not getting older -- just better.