Richard Williams believes daughter Serena needs to get back in shape after suffering her worst-ever form and a shock third-round exit at Wimbledon.
He told BBC Sport: "I think Serena's in the worst condition ever - I've been preaching to her to get in condition."
Williams also admitted tennis was no longer the number one priority for both Venus and Serena.
"They understand that now they're older, there's a lot more to life than just tennis," he added.
Serena missed out on a fourth-round meeting with Venus after losing to Jill Craybas in Saturday's fading light.
The defeat came as both sisters face increasing criticism for being distracted by pursuits outside tennis.
But Williams senior revealed the sport had never been top priority for his daughters.
"Tennis will never be number one in our life - it never has been and never will be," he added.
"God is first and then family, education, business, and then fifth you have to feel good about yourself."
But he still believes there is plenty more to come from the sisters, especially Venus who beat Mary Pierce on Tuesday to claim a place in the semi-finals.
"Serena might have to get into a lot better condition. Serena's actually been hurt a lot - I think she's made a great effort.
"But once Serena's in condition, and Venus, it's kind of hard to beat us."
"Venus will definitely be number one again and she'll win tournaments again, and it's a great possibility she'll win this tournament right here."