Williams, 36, was thrashed 6-1 6-0 by Johanna Konta in only 53 minutes at the Silicon Valley in San Jose.
In a revealing interview with Time magazine, the former world No.1 said she had checked Instagram just 10 minutes before going on court and saw the devastating news.
Her elder half-sister Yetunde Price, who was also personal assistant to Serena and Venus Williams, was the victim of a drive-by shooting in September 2003 in Los Angeles.
Southside Crips gang member Robert Edward Maxfield was sentenced to 15 years in jail but Williams saw that he was already out on parole.
“I couldn’t shake it out of my mind,” said the 23-time Grand Slam champion.
“It was hard because all I think about is her kids and what they meant to me. And how much I love them.
“No matter what, my sister is not coming back for good behaviour. It’s unfair that she’ll never have an opportunity to hug me. The Bible talks about forgiveness.”
Williams was then asked if she forgave the killer, to which she replied: “I’m not there yet.
“I would like to practice what I preach, and teach (her daughter) Olympia that as well
“I want to forgive. I have to get there. I’ll be there.”