Williams went from an unstoppable force early in her Australian Open semi-final clash with Naomi Osaka, to an error-riddled shadow of her former self.
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The 23-time major winner looked to be in irrepressible form as she jumped out to a 2-0 lead, but from there it was a completely different story as Williams made error after error.
She only won one more game in the first set to lose it 6-3, before Osaka stormed into the Australian Open final by winning the second set 6-4.
American great Jim Courier was gobsmacked in commentary, describing Williams' litany of unforced errors as "inexcusable".
Williams went on to make a staggering 24 errors in the match, compared to just 12 winners.
"The difference today was errors. I made so many errors," Williams said in her post-match press conference.
"Honestly, it was opportunities where I could have won.
"I could have been up five-love. I just made so many errors."
Fans and commentators were also left in disbelief after Williams' impressive performances leading into the semi-final.
“It’s just that Williams was playing so well in Melbourne, that she looked closer to her peak self than she had in years, that maybe falling short of a final was more difficult to deal with than she initially expected,” The Athletic’s Christopher Kamrani wrote on Thursday.
Tumaini Carayol of The Guardian believes Williams may have lost her "killer instinct".
“Williams’s biggest problem isn’t her game, but how she has come to lose the killer instinct that defined her for so long,” Carayol wrote.
“She was the great closer. Her ability to excel in the toughest moments was so common that it was easy for people to forget how difficult just winning even one title is. Now we know.
“She has reached the part of her career where experience can be detrimental. She knows too much: she understands exactly what it means to win a grand slam title, that this will all soon be over and that every failure is a wasted opportunity.
"Winning is so difficult under these circumstances.”
Serena Williams breaks down in press conference
Turning 40 in September, Williams's latest quest to match Margaret Court's all-time record of 24 grand slam singles crowns ended in a shattering loss.
With her hand on heart, the American paid a touching tribute to the crowd before leaving Rod Laver Arena, raising the prospect that her 21st Open campaign may have been her last.
"I don't know. If I ever say farewell, I wouldn't tell anyone," Williams said.
"So ... I don't know. The Aussie crowd is so amazing, so it was nice to see."
Williams has been stranded one shy of Court's record since capturing her 23rd grand slam title at Melbourne Park four years ago with victory over her older sister Venus while three months pregnant with her daughter Olympia.
The 39-year-old has since suffered four grand slam final defeats - two at Wimbledon and two at the US Open - and was left devastated by her straight-sets loss to Osaka.
Breaking down in tears, the seven-times Australian Open champion cut short her post-match press conference.
"I don't know," Williams said. "I'm done."
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