In the span of a week, the Beijing Open has gone from two Williamses to zero.

On Friday morning, Venus Williams withdrew from the Beijing Open, citing a left knee injury. The Open begins on Saturday. The 30-year-old Williams reached the semifinals of the US Open three weeks ago but did not compete in this week’s Pan Pacific Open.

She joins sister Serena and world No. 5 Kim Clijsters among those originally committed to the Open who have withdrawn this week.

Even without those three, 16 of the top 20 players in the world will compete in the tournament, which lasts through Oct. 10. The men’s field is led by world No. 2 Novak Djokovic, the defending tournament champion, as well as No. 4 Andy Murray, No. 5 Robin Sodelring and No. 6 Nikolay Davydenko.