They have seen rivals ranging from Jennifer Capriati to Lindsay Davenport to Martina Hingis to Mary Pierce to Kim Clijsters to Justine Henin come and go (and some come back again) and are now approaching age 30  — a milestone mark that has prompted many players to contemplate closing the curtain on their careers.

But both 29-year-old Venus Williams and sister Serena, who celebrated her 28th birthday last month, are not exactly spending their down time devising an exit strategy from the sport.

In fact, on the eve of this week's Sony Ericsson Championships in Doha, the Williams sisters, who qualified for the season-ending event in singles and doubles for the first time, are aiming to extend their careers well into their 30s.

Defending Doha champion Venus says the sisters' recent results — they have combined for six singles final appearances in the last eight Grand Slam tournaments and have won four of the last six Grand Slam doubles events they've entered — as well as the fact they occupy two of the top seven spots in the world point to their plan to prolong their careers.

"Serena and I are playing at a level that would keep us here for quite a few years longer. This is very good news for us," said Venus, who is the oldest player in the top 20, six days older than Moscow champion Francesca Schiavone. "At the end of the year, I'm feel like I'm ending on a high note. I'm at the point in my career where I've been very successful at every tournament just about, almost on every surface. For me it's about obviously playing great tennis, adding to my repertoire, my game, continuing to get better, to enjoy the challenge while being on top."

While elite athletes can seldom anticipate injuries, if the sisters from Compton, California stay healthy, it is conceivable both can remain contenders well into their 30s as they head down the home stretch of their careers.

Here's why:

1. Doubling Up — Billie Jean King once said she saw John McEnroe's status as a Grand Slam singles contender end the moment he stopped playing doubles. "When John announced he wasn't playing doubles anymore, I said 'Oh, Oh, he'll probably be done in a year or two,' " King told Tennis Week in a past interview. "Because he had used doubles as almost a practice to sharpen his game for singles so at the end of his career when he dropped the doubles I felt he wouldn't continue singles much longer." Conversely, the Williams sisters have played more doubles in recent years. They hold three of the four doubles majors — Australian Open, Wimbledon and US Open — as well as the Olympic gold medal in doubles. After their fourth-round exit at the 2003 Wimbledon, the sisters played only one doubles event in the next four years — conceding a walkover in the second round of the 2007 Wimbledon — but in the last two years, the Williams sisters have played 10 doubles tournaments, winning six of them. Doubles generally helps their singles play in that it reinforces some time-honored tennis truths for success: get the first serve in, put the return in play and hit to the open court. Every year they have won a Grand Slam doubles title at least one of the sisters has also claimed a major singles title. Additionally, doubles often serves as a barometer for their desire: when the sisters enter a doubles field it usually means they are confident of sticking around for a while in the singles as well. 

2. Wimbledon — The Championship remain the most coveted title in the game and the Williams sisters have treated Centre Court as if it were their own backyard. The sisters have combined to collect eight of the last 10 Wimbledon singles titles with only Maria Sharapova (2004) and Amelie Mauresmo (2006) breaking their strong hold on the Rosewater Dish. Though former World No. 1 Justine Henin, who gave Serena fits in her last full season before her retirement, has said winning Wimbledon — the only Grand Slam title trophy that has eluded her — is a primary reason for her return to tournament tennis the fact remains there is no true grass-court specialist in women's tennis. The Williams sisters have been so dominant on the All England Club's lawns, they don't even play grass-court tune-up tournaments in preparation. If you knew every year you were a legitimate threat at the start of every season to win Wimbledon, why even contemplate retirement at 30 or 35?

3. Shortened Schedule — Adhering to the "less is more" approach to the calendar, both Williams sisters have scaled down their schedules, in comparison to many other top 10 players, earlier in their career and may reap the rewards with increased longevity. There was a time early in their careers when Venus and Serena dramatically diminished their fall schedules after the US Open as both were taking college courses in their younger years. In fact, both Venus and Serena have played the season-ending Championships just three times apiece. Throughout their careers, the sisters have seldom played more than 16 tournaments a season, leading to less mileage on their legs than contemporaries who have played full seasons year in and year out. The fact that both place a premimum on the majors (Serena's last non-major title came in Charleston in April of 2008; Venus won Dubai and Acapulco back-to-back in February) means they are not going as deep into draws on a weekly basis as other former top-ranked players. Henin, for instance, won 10 of the 14 tournaments she entered in 2007, which was her last full season on the WTA Tour).

4. Serve — It's the most important shot in tennis and both possess two of the most potent serves in women's tennis history. Serena's serve, arguably the best in the history of women’s tennis, has helped her create closure in serving out major championships. In an era where former World No. 1 players including Ana Ivanovic, Jelena Jankovic, Maria Sharapova and World No. 1 Dinara Safina have all struggled on serve and at times hit double digits in double faults, the Williams sisters' serves not only provide a significant edge over top contenders, the serve gives them the opportunity to end points quicker.

There's also the opportunity to add to their legacy in the coming years. 

"I'm feeling good. I actually feel way more energized than I did last year, so that's a good thing," said Serena. "I think as I get older, my priorities change more towards tennis. I want to play more. I want to enter more tournaments. I want to play more doubles. I think my career has been really more focused onto tennis."

Their 10 career doubles majors place Venus and Serena third among the most successful Grand Slam doubles duos in the Open Era. Hall of Famers Martina Navratilova and Pam Shriver won 21 majors. Gigi Fernandez and Natasha Zvereva combined to capture 14 Grand Slam titles followed by the Williams sisters, who could surpass Fernandez and Zvereva for sole possession of second place if they continue to play.

Serena summed up their success simply — picking the right partner has been key.

"I have a great partner. I don't have to work too hard out there," Serena said. "Just hit some big serves. Venus hits some big serves. We put the ball away. Most of all, I love to play doubles. For me it's great practice, great fun. If I'm really fit, then I like to go for the win in both events."