Serena Williams was unable to focus during a recent match as she had just found out on Instagram that her sister's killer had been released from prison.

10 minutes before she was scheduled to face Johanna Konta in July, Williams discovered the news — and went on to suffer the heaviest defeat of her entire 23-year career.

She says she has not yet forgiven the killer.

Serena Williams recently suffered the heaviest defeat of her 23-year professional tennis career last month, and has claimed her poor performance was because she found out her half sister's murderer had been released from prison just moments before she was due to compete.

Williams was beaten 6-1, 6-0 in 53 minutes by Johanna Konta at theMubadala Silicon Valley Classic in San Jose, California, last month.

Williams only managed to win one game in that match — a first in a run that stretched all the way back to her debut in 1995. She made 25 unforced errors, served up seven double faults, and lost 12 games in a row.

Now, it appears she could not focus because of the knowledge that the murderer of her elder half-sister Yetunde Price, who was killed during a drive-by shooting in 2003 in Los Angeles, had just been let out of jail.

"I couldn't shake it out of my mind," Williams told Time magazine in an explosive interview. "It was hard because all I think about is her kids and what they meant to me. And how much I love them."

The murder scene in 2003. AP Images

Robert Edward Maxfield, a reported member of the Southside Crips gang according to LA Times, was sentenced to 15 years but Williams found out he had been released on parole when she checked Instagram, just 10 minutes before she was due to face Konta.

"No matter what, my sister is not coming back for good behaviour," Williams said. "It's unfair that she'll never have an opportunity to hug me. But also… the Bible talks about forgiveness."

Williams was asked if she forgives her half-sister's killer. "I'm not there yet," she said.

The tennis champion's comeback has been no means seamless, and she even opened up about her postnatal depression on social media — a move many people applauded because of her honesty.

Her comeback was also complicated because "everything went bad" after she gave birth. According to CNN earlier this year, Williams endured a "six-day medical crisis" because of blood clots, something she has a history of.

Her "C-section wound reopened after a coughing spell," "she returned to surgery," and "her medical team found a large hematoma," were among the traumas. Another operation followed and she was able to return home after a week.

Regardless, Williams is determined to continue with her comeback. "I'm not done yet," she told Time.