The defending champions - Roger Federer of Switzerland and Maria Sharapova of Russia -  have advanced to the fourth round of the Wimbledon tennis championships in London.

Federer, the top seed, scored a four-set win over Nicolas Kiefer of Germany Saturday, 6-2, 6-7, 6-1, 7-5. The two-time defending champion next faces 2003 French Open winner Juan Carlos Ferrero of Spain, who scored a four-set win over Florian Mayer of Germany (3-6, 6-2, 6-1, 6-1).

Sharapova, the second seed, reached the round of 16 for the third time in her career, 6-2, 6-4. Despite some erratic play, the Russian hit 21 winners, broke Katarina Srebotnik's serve four times and lost her own serve just once.

 Elsewhere in the ladies' draw, eighth-seeded Nadia Petrova became the fifth Russian woman in the round of 16 when she defeated Zimbabwe's Cara Black in straight sets (6-4, 6-3). Petrova takes on Kveta Peschke of the Czech Republic, who knocked 1994 champion Conchita Martinez of Spain out of the grass-court tournament (6-4, 6-1).

French Open runner-up Mary Pierce defeated Ana Ivanovic of Serbia and Montenegro (6-4, 6-1). Pierce will play either Eleni Daniilildou of Greece or Flavia Pennetta of Italy. Two-time champion Venus Williams of the United States ousted Daniela Hantuchova of Slovakia in straight sets, 7-5, 6-3.Venus will play either her younger sister Serena Williams or American countrywoman Jill Craybas.

In men's play, Russian Mikhail Youzhny celebrated his 23rd birthday with a four-set win over Jonas Bjorkman of Sweden (7-5, 6-3, 3-6, 7-6). Youzhny takes on Fernando Gonzalez of Chile in the fourth round.

Some information for this report provided by AP