Both players have right knee ailments.
The event at Carson, Calif. starts Monday.
Williams, who won at Wimbledon this season, is nursing tendinitis and has not played since she beat her sister Serena in the Wimbledon final July 5.
"I have been receiving treatment for tendinitis in my right knee over the last several days and my medical advisers have encouraged me to allow myself an additional week to completely recover," Williams said. "I look forward to returning to tournament play and my plans are still intact to compete (later) this summer."
Davenport's knee injury also prevented her from playing at the French Open.
"I always look forward to the opportunity to play so close to home, but unfortunately my knee has not completely healed at this point," said Davenport, a native of nearby Palos Verdes.