BEIJING - Chinese player Li Na continued her top form in the Olympic Games tennis event as she upset American seventh seed Venus Williams in the quarterfinals of the women's singles on Thursday.

Li Na of China celebrates victory over Venus Williams during their women's singles quarterfinal tennis match at the Beijing 2008 Olympic Games, August 14, 2008. [Agencies]

Venus Williams of the U.S. dries her face during her women's singles quarterfinal tennis match against Li Na of China at the Beijing 2008 Olympic Games, August 14, 2008. [Agencies]

Li, a former world top 20 player who came into the Olympics unseeded, had beaten Russian third seed Svetlana Kuznetsova in the first round of the event.

In an early match, Serena Williams, Venus' younger sister, was also knocked out from the quarterfinals after losing to Russian fifth seed Elena Dementieva.