Serena has not played since losing to Jill Craybas at Wimbledon a month ago.
Shell pulled out of this week's tournament at Stanford and next weeks' event at San Diego with an ankle injury she sustained in early April at a tournament at Amelia Island.
She has only played four matches since then.
"Last week she was trying to do some fitness and this week she's practicing and trying to pick it up," Venus said. "But you can't go from zero to 100 because the body breaks down. But she's going step by step. I think it's definitely in her heart that
On Friday, Venus turned down a wild card to play in next week's Acura Classic in San Diego, saying that she was physically and mentally incapable of playing six straight weeks.
The Tier One tournament has been stung by injury withdrawals from Serena, Maria Sharapova and Justine Henin Hardenne.
"I feel tired," Venus said. "I want to play San Diego and get my ranking up, but I need a break. I wanted to help the tournament out but I have to be realistic. I need to be ready for the U.S. Open."
The U.S. Open starts on Aug. 29.
Lindsay Davenport is the tournament's top seed but she hurt her lower back on Thursday and was forced to pull out of her match against Anna-Lena Groenefeld at Stanford.
She said that she was "below 50-50" to play San Diego.
