ISTANBUL — Serena Williams extended her dominance over the top-ranked player in the world on Thursday, defeating Victoria Azarenka 6-4, 6-4 at the WTA Championships.

ISTANBUL — Serena Williams extended her dominance over the top-ranked player in the world on Thursday, defeating Victoria Azarenka 6-4, 6-4 at the WTA Championships.

Williams improved to 11-1 overall against Azarenka, winning the last nine matches. Williams beat Azarenka in the semifinals of Wimbledon and at the London Olympics.

Despite the loss, Azarenka can secure the No. 1 position with another victory in the round-robin tournament, which features the top eight players.

Williams, who is projected to finish the year at No. 3, won her third match at the event when Azarenka served a double-fault. Azarenka also lost the first set on a double-fault and finished with nine.

Li Na beat Angelique Kerber of Germany 6-4, 6-3 in another Red Group to retain a slim chance of advancing. Sara Errani defeated Samantha Stosur 6-3, 2-6, 6-0 in the White Group.

Swiss Indoors

At Basel, Switzerland, Juan Martin del Potro of Argentina had 10 aces in defeating Brian Baker of the U.S. 7-5, 6-1 to advance to the Swiss Indoors quarterfinals.

Del Potro, seeded second, plays Kevin Anderson of South Africa in today's quarters. Anderson beat Marinko Matosevic of Australia 6-3, 6-7 (5), 6-3.

Top-seeded Roger Federer has a quarterfinal against Benoit Paire of France.

Valencia Open

At Valencia, Spain, David Ferrer cruised to a 6-4, 6-2 victory over Albert Ramos to reach the quarterfinals of the Valencia Open.

Ferrer, seeded No. 1 at the indoor tournament, served well and broke his 47th-ranked opponent four times in a much easier match after a labored first-round victory. The fifth-ranked Spaniard next faces Nicolas Almagro, who defeated American Sam Querrey 6-4, 6-7 (4), 6-0.

Marcel Granollers extended his dominance of Gilles Simon with a 6-7 (4), 6-3, 6-2 victory that was the reigning champion's fourth victory in five career meetings.

Granollers plays Alexandr Dolgopolov in the quarters after the Ukrainian beat Gilles Muller of Luxembourg 6-4, 6-4. Jurgen beat Xavier Malisse 7-5, 6-2.

AP-WF-10-25-12 2123GMT