~ The sport's most prestigious tournament begins today.

WIMBLEDON, England -- Who says the oldest and most tradition-laden of tennis' major championships doesn't change with the times?

There is, at long last, a retractable roof atop Centre Court at Wimbledon this year. Video review of line calls and equal prize money for men and women came along ages ago (well, OK, in 2007).

Rest assured: They still use grass courts, they still make the players wear white, and they still schedule a day off on the middle Sunday of a tournament first held in 1877. Here's something else that stays the same at the All England Lawn Tennis and Croquet Club: Venus Williams and Serena Williams are the women to beat.

Venus, in particular. She's won five titles, including the last two, at Wimbledon, which begins Monday. Serena has won this Grand Slam tournament twice, beating Venus in the 2002 and 2003 finals and losing to her in last year's championship match.

The names and faces at the top of women's tennis keep switching, as players emerge, then recede or retire -- Martina Hingis, Justine Henin, Jennifer Capriati, Lindsay Davenport, Amelie Mauresmo. The Williams sisters? One or the other -- or both -- participated in eight of the past nine Wimbledon finals, and they're the top picks of British oddsmakers this time.

As the defending women's champion, No. 3-seeded Venus is slated to play her first match Tuesday on Centre Court, against Stefanie Voegele of Switzerland. A statistic for Voegele to ponder: Her career Grand Slam record is 0-1, and Venus' is 180-38.

Second-seeded Serena starts today. She faces 154th-ranked Neuza Silva of Portugal on Centre Court after Roger Federer meets Yen-hsun Lee of Taiwan in what, in theory, could be the first match in the 132-year history of Wimbledon to be played indoors. There is a 20 percent chance of rain today.

The new translucent roof on Centre Court, an arena that dates to 1922, takes about 10 minutes to close and should forever eliminate those days when zero tennis is played.

Federer begins his bid for a sixth Wimbledon championship, and a record-breaking 15th major title, a couple of weeks after completing a career Grand Slam by winning the French Open.

Neither Williams fared that well in Paris, with Venus losing in the third round and Serena in the quarterfinals. Venus also was upset in the second round at the Australian Open in January, and she's won only five of her past nine matches.

But form never seems to matter for the sisters when the season shifts from clay to grass, even if they opt to skip tuneup tournaments between the French Open and Wimbledon. In 2008, for example, both lost in the third round at Roland Garros, then forged an all-in-the-family final at the All England Club.

Venus' Grand Slam trophy collection was amassed mainly at Wimbledon; she won the U.S. Open in 2000-01. Serena, meanwhile, has at least one title from each of the other three majors, for a total of 10. Venus was asked Sunday why Wimbledon is by far her best Grand Slam tournament.

"Everyone asks that. My answer is: why not?" Venus said. "Everybody wants to know the secret. There is no secret."

A few moments later, she was slightly more forthcoming, saying: "You're rewarded for playing aggressively, and that's definitely how I play: aggressively. I have lots of rewards."