LONDON -- When you're Serena Williams, there is one telltale sign that you're getting older, and it doesn't involve AARP cards or reading glasses or heartburn.
"It's strange," she said this week, "because a lot of the people I grew up with are now on the Legends Tour. It's like, 'Should I play Legends as well as the main?' I'm like: 'Did I play that lady? Yeah, I did.' So it's actually kind of funny. Me and Venus [her 35-year-old sister] just laugh, 'Oh my gosh, we played her.' "
Of course, no one else on the WTA Tour is laughing much unless, perhaps, they are thinking Serena may really join the Legends Tour soon.
But then that thinking could be dangerous.
At 33 years and nearly 10 months old, Serena Williams is aiming to become the oldest Grand Slam winner in the Open era. With a victory Saturday over Spain's Garbine Muguruza in the Wimbledon final, Williams would surpass Martina Navratilova, who won Wimbledon in 1990 at 33 years, 263 days (Serena will be 33 years, 289 days on Saturday).
Former players marvel at Williams' longevity and a 20-year career that has produced 20 Grand Slam titles, including the past three. But more than the sheer numbers are what those titles represent in physical stamina and motivation.
Andy Roddick, whose time on the ATP Tour spanned the bulk of Williams' career, grinned when talking about his old friend.
"I remember it was '07, '08, when she had missed a whole bunch of time and she had dropped out of the top 100," said the 2003 US Open champion, who announced his retirement on his 30th birthday in 2012. "I was still sitting there at [No.] 3 or 4 in the world and I'm going, 'Gosh, she's giving up years. I don't understand how she's not out there,' whether it was a little bit physical, a little bit mental.
"Now fast-forward and I've been retired for three years and she's still winning majors. So I look back and kind of think to myself that she must have had it figured out all along."
Like Roddick, former champions Mats Wilander and even Chris Evert, who won two of her 18 Grand Slam singles titles -- both at the French Open -- at ages 30 and 31, described the biggest obstacles in overcoming age as more mental than physical.
"Because physically," Evert said, "between 25 and 35, you should be getting into your best shape anyway. I know I did. But mentally, just waking up in the morning and wanting to train, waking up and looking forward to playing a match, being eager, being fresh, being excited, that's what it is.
"For [Serena and Venus], it's almost 20 years, and after 15, 20 years, when I was like 32, that's when I decided, 'OK, one of every four matches, I'm not motivated.' That was the first thing to go."
Wilander, a seven-time Grand Slam singles champion who won his last at age 24 and retired at 32, agreed, saying aging in sports is "all motivation-based."
"The physical part is easy, the practicing part," he said. "It's just, 'Why am I doing this?' The difficult part for me or for anyone, I think, is your motivation has to keep changing along the way. Like sometimes you're motivated to win. Win another major. Stay at No. 1. Or to stay healthy. I don't think you can have motivation as long as [Serena] has [just] to try and catch Steffi Graf [and her Open era record of 22 Grand Slam singles titles]. That doesn't work, I don't think."
Williams said there is not one thing that has necessarily become easier as she has gotten older, but she is less on-edge about winning. "I just think I don't feel as desperate to win ...," she said. "Like, I was so desperate to get to 18 [tying Evert and Navratilova], and ever since then, I've been totally relaxed."
Patrick Mouratoglou, Williams' coach since 2012, doesn't disagree. But as Williams gets older, he said he believes it does become easier for her to appreciate her career.
"She's closer to the end because she's 33," Mouratoglou said Friday. "And because she feels she's getting close to the end, she wants to enjoy it. She realizes what she's going to lose after, and how great this journey is for every tennis player but even more for her with what she can achieve. So I think that's also one of the reasons why she's so motivated and focused."
Since she turned 30, Williams has won seven Grand Slam titles, four more than Margaret Court and Navratilova, who are next on the list. And Mouratoglou attributed the wisdom and maturity that often come with age for helping sustain her success.
"She knows very much what she wants," he said. "She knows how important tennis is for her and for her life, so she puts all the effort into it. Also, I think with age she understands the game better, she got more interested in the game. She's watching a lot of tennis, a lot of men's tennis, to progress and to understand better. So I think she's a more complete player also.
"So if you add those things to the qualities she had -- the best serve of all time, probably the most powerful player of all time and probably the best player mentally also of all time -- and add focus and consistency every day, understanding of the game and motivation, it's all there."
Yet that still does not account for the physical reality of the aging process. "You take longer to recover, there's no two ways about it," said Virginia Wade, a three-time Slam winner whose tennis career spanned 26 years, as he retired from singles at 40 and doubles at 41.
Pam Shriver, who served as a WTA mentor to Venus Williams in the late '90s, remembers telling her and Serena's father, Richard, that they should hire a physiotherapist full time, now a common member of top players' teams. Shriver also said Serena's recent interest in team dance has become a cross-training element in her workout and a way to keep her training fun.
"It's another way. It doesn't have to be boring," Mouratoglou agreed. "And I think if it's not, you do it much better. ... So dancing, or pole-dancing, it's incredibly physical, but she takes so much pleasure in it, so she does a lot and she does it well and I think it keeps her level of athleticism at a level that is great."
It didn't hurt that both Serena and Venus had prolonged breaks during their careers, mostly because of injury and illness but often depicted as a reflection of their waning interest in the game.
Mouratoglou acknowledged that may have saved Serena's legs and mental energy, but added: "If you look at Roger [Federer], he didn't stop and he's still just as motivated. Both of them just love the game. But they also love to be at the top of the game, which they realize how incredible it is. And the older you get, the more you realize that, I think.
"She doesn't want to lose that. She wants to keep this journey at the top as much as she can. That's definitely the goal. And then if she breaks records, that's great."
Wilander and others theorized that Serena's hematoma and pulmonary embolism in early 2011 as a result of stepping on glass following her 2010 Wimbledon title -- a life-threatening condition that kept her off the tour for nearly a year -- also gave her added motivation in the latter stage of her career.
"I think it made her think, 'Wow, this is really a fun life,' " Wilander said.
"Serena had to restart her career often, which is very difficult, of course. But I think it must really give you natural motivation to think: 'Man, I want to do it again. I want to start again.' She stayed fresh and I think a lot of people were too early in criticizing the Williams sisters for not taking it as seriously as they should.
"[Critics said]: 'Oh, they take off and they're doing their sewing and their acting and why can't they focus on tennis?' But in the meantime, look at them. Here they are at 33 and 35. I don't think they knew what they were doing, necessarily, but it turned out that it was absolutely the right thing to do."
Evert said Serena was "lucky" to find Mouratoglou, both as her coach and partner. "Three summers ago, she was flailing a little bit," Evert said. "She lost the first round of the French and she got a sort of new, exciting change of scenery, change of coaches, change of lifestyle. You have to set goals for yourself and you have to have things to look forward to. ...
"You need somebody that inspires you, too, and he inspired her. And now her incentive is the records."
Shriver pointed out the mental exhaustion that can come from always being the chased, but now that Serena is doing the chasing, her main motivation seems obvious.
Despite fending off any questions here that mention her quest for the "Serena Slam" (holding all four Slam titles concurrently, which she would accomplish with the Wimbledon title) and calendar-year Grand Slam (which she would complete with Wimbledon and US Open titles), she could not deny it.
"I mean, right now, it's definitely the obvious," she admitted. "It was never my goal. But right now it's kind of cool. So that completely gets me motivated to work harder."
More than that, said her coach, is simply a deep desire to be the best that keeps Williams young. The number is unimportant.
"I don't think she's thinking about that," he said. "She wants to do well. Even if she was 53, she would want to do well. If she feels she can't, then she would stop. But she feels she can. She feels that she has the level. She feels she can beat those girls. And she feels that she can win these tournaments.
"She doesn't think about the age she is, she thinks about what she can achieve."