The American former number one said she had pulled out because she did not want to harm her chances at the French Open starting later this month and the knee problem was linked to a thigh injury that has been bothering her in recent weeks.
In the men's draw, ninth seeded Frenchman Jo-Wilfried Tsonga battled past his former idol Marat Safin 6-4 7-5 to set up a second-round clash with Croatian wild card Ivan Ljubicic.
Tsonga, 24, said he remembered watching the 29-year-old Safin, a former world number one, playing at the French Open and the Paris Masters, where the Russian won three times and finished runner-up once.
"For me he was like an idol before so it was really special to play against him and to win against him," Tsonga told a news conference.
"I liked his game but before everything his personality," he added. "Today I was happy with my game. I played great tennis, some good serves and volleys, and I was really aggressive so I'm happy today."
Twelfth seed David Ferrer of Spain beat Guillermo Canas of Argentina 6-2 6-2 and 11th seed Stanislas Wawrinka of Switzerland thrashed Italian qualifier Marco Crugnola 6-2 6-0.
In the women's draw, Russian third seed Elena Dementieva was due to play Lourdes Dominguez Lino of Spain later on Monday.
Men's top seed and world number one Rafael Nadal begins his his quest for a sixth title of the year, and a fourth on clay, later this week against the winner of the first-round clash between Juergen Melzer and Albert Montanes.
Women's world number one Dinara Safina, who also had a bye into the second round, is due to play China's Li Na for a place in the last 16.
