Beijing, China (My Sportsbook) - In her first match since regaining the world No. 1 ranking, Serena Williams suffered a third-round defeat at the hands of Russian Nadia Petrova Thursday at the $4.5 million China Open.

The 13th-seeded Petrova outlasted the second-seeded Williams 6-4, 3-6, 7-6 (7-5) at the Olympic Green Tennis Center. Petrova pulled off the upset in 2 hours, 19 minutes on Day 6 and left the big event without any of its' top- three seeds.

On Tuesday, the reigning Wimbledon and Australian Open titlist Williams beat Russian Ekaterina Makarova to reclaim the top ranking from French Open and Aussie Open runner-up Dinara Safina.

Williams, who captured this Beijing event back in 2004, will officially become the new No. 1 when the women's rankings are released on Monday.

A trio of seeds avoided upsets on Thursday, as No. 7 Russian Vera Zvonareva snuck past 10th-seeded Italian Flavia Pennetta 6-3, 5-7, 7-5; No. 12 Pole Agnieszka Radwanska came back to beat Spaniard Maria Jose Martinez Sanchez 4-6, 6-4, 6-2; and No. 14 Frenchwoman Marion Bartoli doused 20-year-old Chinese wild card Zhang Shuai 6-1, 6-4. The 226th-ranked Zhang shocked a top- seeded Safina here on Monday.

Friday's quarterfinals will pit fourth-seeded Russian Elena Dementieva against Radwanska, sixth-seeded Svetlana Kuznetsova versus fellow Russian Anastasia Pavlyuchenkova, Zvonareva against Bartoli and Petrova versus giant-killer Peng Shuai, who shocked former world No. 1 and defending Beijing champion Jelena Jankovic in the second round and former top-ranked star and last week's Tokyo titlist Maria Sharapova in the third round this week.

Dementieva captured an Olympic gold medal at this same tennis complex 14 months ago, while Kuznetsova was last year's China Open runner- up to Jankovic, titled here in 2006 and was also the runner-up here, to Williams, back in 2004.

The 2009 Beijing champion will collect a whopping $775,500.