McGuire said in a written statement that there was no reason to believe the family had conspired to predetermine who would win matches between the siblings prior to, or without, going on court.
"The Tour is aware of assertions being circulated regarding Venus and Serena Williams head to head matches," McGuire said.
"We have seen no evidence to support those assertions and both players have denied them."
After Venus Williams withdrew from the semi-final with a sore knee, some observers speculated that Venus had been told by her father and coach Richard to give the victory to Serena.
Both players immediately dismissed the speculation as unfounded rumours.
"Everyone makes their own comments and that's how rumours get started.
"I guess rumours are more exciting than the truth," Venus said after pulling out of the match.
The usually talkative Richard Williams, who has a penchant for making outrageous comments simply for shock value, would not even address the issue.
"I don't want to open my month anymore. Every time I do, all that's printed is lies," he said.
"I'm scared. I'll never talk again. It's all lies. I don't speak English anymore."
Russian player Elena Dementieva jump-started old rumours about the legitimacy of Williams-Williams contests when she said that it would be Richard Williams' decision as to who would win through to the final.
Both Martina Hingis and Lindsay Davenport have previously revealed their reservations about past matches between the superstar sisters.
But McGuire emphasised the Williams sisters competitive desire over their sense of protecting each other's egos or desire to please their father.
"They are elite competitors who are driven to win," McGuire said.
"Along with other siblings who have played against each other, Venus and Serena have always said that head to head matches are difficult.
But he concluded: "Their competitive instincts and pride transcend their sense of family when they play each other."