Serena Williams says she discovered that the man convicted of killing her half-sister had been released on parole just minutes before her match with Johanna Konta last month.
Briton Konta inflicted the worst defeat of Williams' career, winning 6-1 6-0 in the first round of the Silicon Valley Classic in San Jose.
Williams' elder half-sister Yetunde Price was shot dead in 2003.
"I couldn't shake it out of my mind," said the 23-time Grand Slam champion.
Robert Maxfield was jailed for 15 years in April 2006 and Williams, 36, attended the sentencing.