The Madrid Masters will provide Rafael Nadal with a difficult final warmup ahead of the French Open.
The top-ranked Nadal, who will be going for a record fifth straight title at Roland Garros later this month, leads the new-look Madrid tournament, which has been transformed from a men's hard court event into a clay event _ the surface that the Spaniard loves best.
And although he carries a 30-match unbeaten clay run, Nadal's not a big fan of the Spanish capital's altitude, which could negate his advantage in long rallies with high bounce and provide a possible upset.
Madrid's 600-meter (2,000-feet) altitude provide challengers Roger Federer, Andy Murray and Novak Djokovic some hope going into the euro 3.7 million ($4.95 million) event.
Serena and Venus Williams and top-ranked Dinara Safina lead the women's side of the bracket, which will be without injured Ana Ivanovic and Maria Sharapova.
Women's play was to start on Saturday and the men on Sunday. Nadal has won five titles from six finals this season and comes to Madrid off clay wins at Monte Carlo, Barcelona and Rome. Most impressively, the 22-year-old Mallorcan can move to within one of Andre Agassi's record Masters Series titles haul of 17, in front of a home crowd. It would also be the first time sine 1990 that a player has won all three clay court Masters titles in one season.
"Now is the time for him to let someone else win a tournament," said Fernando Verdasco, who was drawn to potentially meet Nadal in the quarterfinals.
Federer is defending 700 rankings points, with Djokovic and Murray poised to take over his No 2 ranking due to the Swiss star's lost momentum since the Australian Open final loss to Nadal. Murray is playing his first event since taking over the No 3 spot from Djokovic.
Serena Williams will be looking to snap a three-match losing streak and prove she is in fact the "real No 1" as she proclaimed at Rome before losing in the first round, while the current No 1 Safina will face Svetlana Kuznetsova in the final on Saturday. Williams' first test is Italy's Francesca Schiavone while Safina gets a bye into second round, where a tough match against Spanish player Carla Suarez Navarro could await.
The Williams sisters could meet in the quarterfinals of the $4.5 million (euro3.3 million) event, where nearly every player in the top-20 is playing, including Jelena Jankovic.
One of the biggest attractions will be the state-of-the-art "Magic Box" stadium designed by French architect Dominique Perrault. It features retractable roofs over the three main stadiums, with center court holding a capacity 12,500 spectators. The site underwent an inspection by the International Olympic Committee, as it is a key feature of Madrid's 2016 Olympics bid.