“Thirty-fun.”

That’s how Serena Williams referred to her age in post-match interviews on Saturday. Only one number was on her mind earlier that day as she took the court to face her sister Venus in the Australian Open final. It wasn’t her age — which is 35, by the way. It wasn’t the number one — the ranking she could steal back from Angelique Kerber, who had made an early exit in Melbourne, Australia. It was one magic number, narrowly missed by Steffi Graf, untouched by any other in the Open era, and finally achieved by Williams, the 2017 Australian Open champion: 23. The 23rd victory wave. The 23rd shining silver chalice. Her 23rd Grand Slam title. A number that makes Williams the winningest player in the Open era. A number that knows no age limit.

This year’s Australian Open finals were dominated by players over 30, making longevity the story of the tournament. Not only did the women’s final feature the greatest sister act in sports, 35-year-old Serena and 36-year-old Venus Williams, the men’s final reprised a legendary rivalry, pinning Roger Federer, 35, against Rafael Nadal, 30. In men’s doubles, the 38-year-old Bryan brothers fell in a narrowly contested final.

These players, particularly Serena Williams, are presenting a guide to decades-long professional careers for their peers, keeping fans glued to the TV watching their favorite stars and staying active on the court chasing the example of lifelong fitness.

Tennis’s modern-day 30-something stars are a class all their own. Their age is not unprecedented. The black-and-white images of tennis’s yesteryear feature many players over 30 chipping and charging with wooden racquets. Billie Jean King played singles until she was 40 and continued in doubles for almost another decade.

But the current generation stands out because they have achieved this longevity in the graphite-racquet era, which has made footwork, speed and cardiovascular endurance some of the game’s biggest weapons.

When graphite racquets came on the scene in the 1980s, two decades of youth dominance ensued. Monica Seles won her first grand slam in 1990 at age 16 and became number one at age 17. Older players often could not match the raw athleticism necessary to keep up for hours-long-split-set showdowns.

The greats started to leave the game right around age 30. Steffi Graf, who held 22 titles and the record for most slams in the Open era before she was surpassed by Serena Williams, retired at age 30. Hardly any players on the men’s or women’s side over the age of 30 won Grand Slams for a decade and a half from 1990–2005.

But what fans, commentators and perhaps even the players themselves did not expect was that the stars of the early 2000s were just starting to enter their golden years. This became startlingly clear after the 2012 Wimbledon finals. Serena Williams and Roger Federer both ended long droughts to hoist the golden cup, marking the first Grand Slam won by a woman over 30 since 1990 and by a man over 30 since 2003. Since then, these golden oldies have won a combined 10 Grand Slams.

A new era of off-court training in tennis is largely the source of this success. Players are devoting an unprecedented amount of time to weight-lifting, speed training and other conditioning pursuits that have equipped them to last not just hours, but years on the court.

Today’s stars are also meticulous about planning their schedules, taking time off whenever necessary so that they are mentally and physically fresh for the slams. Williams played only eight events in 2016. Kerber played 21. Some focused solely on the failure of her season — the fact that she surrendered her number one ranking to Kerber. But many failed to realize the feat of playing only eight events, managing to remain the world number two in the world, winning one grand slam and reaching the finals of the Australian and French Opens along with the semifinals of the U.S. Open. Not only did Williams add to her trophy count, she allowed herself to stay fresh for what will certainly be another immensely successful season in 2017 — her 22nd year on the pro tour.

For her fellow pros, her career provides the archetype for a long career that is successful not only in terms of her trophy count and bank account, but also one that has allowed her to maintain a true love for the game.

For fans, players like the Williams, Bryans, Nadal and Federer allow them to witness their favorite moments over and over: Federer’s jaw dropping in a joyous scream after that final shot, Nadal’s thunderous reverse forehand, the classic Bryan chest bump and the Williams sisters’ long embrace.

But it’s about more than just nostalgia. These players allow fans to believe that persistence and perseverance are possible at any age. Serena Williams best summed up that sentiment in a post-match interview.

“It really just proves that as long as you keep going and you keep fighting and never give up and just never stop, you can conquer anything that you want to.”