NEW YORK (CBSNewYork) — Tennis great Serena Williams is opening up about one of the toughest days in her career, saying sorry for blowing up on the court while losing a US Open Final.

It was a day of pure frustration for Williams at the 2018 US Open Final, CBS2’s Dick Brennan reports. She was pitted against 20-year-old sensation Naomi Osaka, of Japan, and, for a good portion of the match, the umpire.

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“You will never, ever, ever be on another court of mine as long as you live. You are the liar,” she said during the final.

Serena Williams argues with referee Brian Earley during her Women’s Singles finals match against Naomi Osaka at the 2018 US Open on Sept.8, 2018, in New York (credit: Jaime Lawson/Getty Images)

It boiled over when the umpire accused Williams of being illegally coached from the stands. Williams denied cheating.

“You owe me an apology,” she told the umpire.

Williams further unraveled when she was ultimately penalized a game and went on to lose the match to Osaka. The youngster even apologized for winning.

“I’m sorry it had to end like this,” Osaka said.

NEW YORK, NY – SEPTEMBER 08: Naomi Osaka of Japan after winning the Women’s Singles finals match alongside runner up Serena Williams of the United States on Day Thirteen of the 2018 US Open at the USTA Billie Jean King National Tennis Center on September 8, 2018 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Julian Finney/Getty Images)

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Now, in an article in Harper’s Bazaar, Williams says she did the apologizing, too.

She said after her meltdown, days passed and she couldn’t find peace. She even saw a therapist, but she finally reached out to Osaka by writing to her.

“I am so proud of you and I am truly sorry. I thought I was doing the right thing in sticking up for myself. But I had no idea the media would pit us against each other,” Williams wrote.

Osaka graciously responded.

“People can misunderstand anger for strength because they can’t differentiate between the two,” she said. “No one has stood up for themselves the way you have and you need to continue trailblazing.”

Williams says when she got that response from Osaka, tears rolled down her face.

“This incident… exemplified how thousands of women in every area of the workforce are treated every day,” Williams said. “We are told to sit down and be quiet, which frankly is just not something I’m OK with.”

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Williams says now she won’t ever stop raising her voice against injustice, adding her daughter is the reason she uses her voice and the reason she picked up her racket again.