U.S. Open 2013: Li Na, 31, is one of several players on the women’s side who are excelling deeper into their careers thanks to innovative training methods. (Julio Cortez/Associated Press)

With her results increasingly erratic and her world-ranking slipping in the 14 months following her 2011 French Open triumph, Li Na fired her husband as her coach and sought out someone to push her harder.

After Day One of the grueling six-hour fitness regimen installed by Carlos Rodriguez, Li was excited, confident results would follow.

After Day Three, she recalled earlier this year, “I was dying!” She phoned her husband and told him Rodriguez was “crazy” to demand as many hours sweating in the gym as practicing on court. She even contemplated retiring.

But today, a full year into the regimen, the 31-year-old Li boasts greater strength and stamina, as well as a more potent serve and aggressive tactics. On Friday, she’ll face a fellow 31-year-old, top-ranked Serena Williams, in the semifinals of the U.S. Open.

By all accounts, both women are in the best shape of their lives at 31 and, as a result, playing the best tennis of their careers.

While exceptional athletes, to be sure, Williams and Li aren’t necessarily aberrations at the top ranks of women’s tennis today. Instead, they reflect a trend in which pro careers blossom later and last far longer than they did two or three decades ago, when teenagers such as Tracy Austin, Monica Seles and Martina Hingis won the U.S. Open women’s title before turning 18.

Of the eight women to reach the U.S. Open’s quarterfinals this year, a record five are 30 or older: Williams, a four-time champion; Li, reveling in her best showing in the event to date; 30-year-old Daniela Hantuchova ; and Flavia Pennetta , 31, who defeated Roberta Vinci , 30, Wednesday to reach the semifinals of a major for the first time.

Williams was the last player under 18 to win the U.S. Open, claiming the first of her 16 major titles at age 17, in 1999.

Today, three weeks shy of her 32nd birthday, she is crushing everyone in her path, sailing into the U.S. Open’s semifinals without surrendering a set and competing in doubles with her older sister Venus.

There are multiple factors driving the new longevity in women’s tennis.

Chief among them, many believe, is the Women’s Tennis Association’s decision, in 1995, to limit the number of pro tournaments that teenage girls may enter, effectively delaying the start of the rigorous grind in store.

Under the so-called “age-eligibility requirements,” 14-year-olds are limited to competing in eight pro tournaments. The permissible number of events gradually increases each year until age 18, when all limitations are lifted.

The move was prompted by concern over the high burnout-rate of promising youngsters who turned pro in their middle-school years, often pushed by parents with grandiose dreams and scant patience.

A 2004 study conducted on the 10-year anniversary of the age-limits rule found that pro careers were lasting 24 percent longer.

The increased prize money and endorsements in women’s tennis has also extended pro careers, Martina Navratilova believes.

For starters, today’s pros can afford to hire first-rate athletic trainers and physical therapists to help keep them in shape and recover from injury. Moreover, the money in itself is a compelling reason to keep playing.

“Their income makes it possible to take better care of themselves and delay that second career,” says Navratilova, 56, who won 18 Grand Slam singles titles in her Hall of Fame career. “What else are you going to do that pays you as much money? What else are you going to be as good at as you are in tennis if you’re among the top 10 or 20 in the world?”

According to a report in last month’s Forbes magazine, tennis players Maria Sharapova ($29 million), Serena Williams ($20.5 million) and Li ($18.2 million) were the top three highest paid female athletes in the world last year.

Navratilova pioneered the adoption of rigorous fitness and conditioning in women’s tennis. But she didn’t start until age 26, after she had won the first three of her 18 majors, urged on by eventual Basketball Hall of Famer Nancy Lieberman, who had trained far harder as a star at Old Dominion than Navratilova ever had on the pro tennis circuit.

“She said, ‘What are you doing? Why aren’t you doing more? You’re wasting your talent! You’re not training nearly as hard as you could or should!’” Navratilova recalls. “It wasn’t that I was lazy. I just didn’t know. She got me off my bum.”

Like Li, Williams trains extensively in the gym. But Williams has also benefited from extended breaks during her 18-year pro career, whether because of injury or personal reasons.

Williams also credits her father’s unconventional methods with lengthening her career. Despite any lack of formal coaching training, Richard Williams molded daughters Venus and Serena into world champions, stressing lightning quick footwork and aggressive tactics in daily drills on the public courts of Compton, Calif., while holding them out of the high-pressure junior circuit that often beats up youngsters’ confidence.

“He’s just so innovative,” Serena said of her father this week. “I think one of the reasons I’m still playing some of the best tennis at 30 is because he built my game and my sister’s game. He gave us a good foundation. It was solid. It wasn’t weak. So we were always able to grow our game.”