Serena Williams, turning 31 this month, is a force in women’s tennis. (Al Bello/Getty Images)

When the draw was unveiled for the 2012 U.S. Open, there was little debate over who was favored: Five-time champion Roger Federer on the men’s side, and three-time championSerena Williams on the women’s side.

The fact that they’re 31 and 30, respectively, barely drew comment.

To be sure, Federer and Williams are the most dominant players of their generation, if not the best ever, so it’s foolhardy to draw generalizations from their success. But on the matter of longevity, they’re emblematic of a trend at the top ranks of tennis, which is increasingly populated by players who have extended their careers to peak at ages when top pros a few decades ago had long since retired.

At this year’s U.S. Open, four of the men who reached the round of 16 were 30 or older: Federer,Spain’s David Ferrer and Americans Mardy Fish and Andy Roddick.

And Thursday afternoon’s most compelling match was the gut-spilling, five-set quarterfinal between Ferrer and Janko Tipsarevic, who are playing their best tennis at 30 and 28, respectively.

On the women’s side, Williams’s dominance has surprised no one. With her 31st birthday looming Sept. 26, she has steamrolled every challenger in recent months when coveted trophies were at stake, winning her fifth Wimbledon in July, claiming singles and doubles gold at the London Olympics in August and storming into the semifinals here without surrendering a set.

But few anticipated the strong run of 29-year-old Roberta Vinci of Italy, who upset the tournament’s No. 2 seed, Agnieszka Radwanska, 23, in straight sets to reach her first Grand Slam quarterfinal. That’s more than twice Tracy Austin’s age (14) when she won her first pro tournament in pigtails and pinafores. And it’s 13 years older than Austin was (16) upon reaching No. 1 in the world, only to have her promising career effectively ended at 20 by a serious back injury.

Asked why she felt she had reached her prime at her age, Vinci replied: “Well, probably because I’m 29. I’m not young. I have a lot of matches behind me, a lot of experience.”

It’s not as if past generations didn’t boast the occasional defiant lion who turned back the clock. The 39-year-old Jimmy Connors was cheered by aging duffers everywhere for his fist-pumping charge to the U.S. Open semifinals in 1991. Andre Agassi burnished his Hall of Fame legacy with his late-career resurgence, winning two of his eight majors after turning 30 and staving off retirement until 36. And Martina Navratilova, who won 18 Grand Slam singles titles, competed in doubles until reaching 50.

Among the first tennis champions to recognize the value of a structured fitness program, Navratilova says she is thrilled to see more players on both the men’s and women’s side extending their careers. She cites many factors as driving the “successful aging.”

“Everybody is getting better care; that’s the bottom line,” Navratilova said in a telephone interview. “It’s not that the bodies are that much better. We have better nutrition at an earlier age. And even though they’re hitting the ball harder and playing on harder surfaces, they can stay ahead of the curve with better training.”

Mark Kovacs, a strength and conditioning specialist who serves as a consultant to the U.S. Tennis Association, says top players typically devote as much, if not more, time to off-court training than they do tennis-specific drills. Much of that off-court work involves exercises geared toward preventing injury — strengthening the lower back and core abdominal muscles that take a pounding over the course of a tennis career.

Contemporary tennis pros are more knowledgeable and mindful of proper nutrition — not only eating the ideal mix of proteins, carbohydrates and fats, but also hydrating sufficiently before matches and refueling properly after matches.

While nearly every player has his own routine, often a guarded secret, Kovacs says players need to consume 200 to 400 calories (roughly 75 percent carbohydrates and 25 percent proteins) within 45 minutes of finishing matches to help their bodies bounce back.

“It speeds the recovery process and helps the body stay in the growth stage rather than the breakdown stage,” Kovacs says. “If they don’t consume that, the muscle breaks down, and that’s not what they want.”

Proper recovery also includes thorough stretching, a cool-down routine and a full meal within two hours of finishing a match.

Still, the sport simply exacts a harsher toll on some players than it does others.

Federer glides around the court with a dancer’s fluidity and sprinter’s efficiency. Rafael Nadal thunders, his every movement and stroke the product of maximum labor. Nadal has twice been sidelined by knee ailments: one caused him to miss Wimbledon in 2009; this year, he was forced to miss the Olympics, U.S. Open and Spain’s upcoming Davis Cup match.

That’s why former coach Brad Gilbert believes that Nadal at 26 is “older,” in terms of mileage on his body and joints, than Federer at 31.

It’s a valid point, Kovacs says.

“It takes more out of an athlete if you’re not as efficient in your movement,” Kovacs says. “It does potentially shorten a career.”

Noted Roddick, discussing his decision to retire at this year’s U.S. Open: “Wear and tear and miles is something that’s not really an age-thing.”

Smart scheduling, conversely, can extend careers. Federer plays in fewer tournaments than most of his rivals. Williams has competed even more sparingly, partly due to injury and illness in recent years. But it has helped prolonged her career and preserved her love of competing, which is as palpable at 30 as it was at age 19, when she won her first U.S. Open in 1999.

“First of all, Serena is an amazing physical specimen,” Navratilova says. “She’s naturally strong, and works at it, as well. She doesn’t wake up that way; she goes to the gym and does her work. And the fact that she hasn’t played that many matches helps her. I played twice as many singles matches at age 30 than she did, so she is fresher at 30 than I was. She’ll be able to play longer if she wants.”