Serena Williams says she made "far too many errors to be a professional athlete" as she was knocked out in the Australian Open third round.
The 38-year-old American lost 6-4 6-7 (2-7) 7-5 to China's 27th seed Wang Qiang.
The defeat means Williams' quest for a 24th Grand Slam singles title goes on.
"I can't play like that. I literally can't do that again. That's unprofessional. It's not cool," Williams said.
The seven-time champion in Melbourne beat 28-year-old Wang in just 44 minutes at the US Open in September when the Chinese player won only one game and 15 points.
That was not reflective of the ability of a player who has been a regular fixture in the world's top 30 over the past 18 months and reached a career high of 12 after her run in New York.
This time, after initially flinching when serving for the match at 5-4 in the second set, she made amends in spectacular fashion.
Wang had the tools to hurt Williams and continued to execute them in a third set in which most observers would have backed her illustrious opponent to go on and win.
After briefly faltering again as Williams rallied, Wang sealed victory on her third match point.
"I honestly didn't think I was going to lose that match," said Williams when asked on her thoughts after levelling at one set all.
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Williams had not lost so early in Melbourne since falling to Daniela Hantuchova at the same stage in 2006
Williams, seeded eighth, came into the match on the back of winning the Auckland Classic and relatively straightforward wins over Russia's Anastasia Potapova and Slovakia's Tamara Zidansek.
Instead of those results laying the platform for another title challenge, they preceded her earliest exit at the Australian Open since 2006.
"I made a lot of errors. I didn't hit any of those shots in New York or in general in a really long time," Williams said.
"I just made far too many errors to be a professional athlete today.
"I'm definitely going to be training tomorrow. That's first and foremost, to make sure I don't do this again."
Opportunities to equal Margaret Court's record of Grand Slam singles titles are running out for Williams, who is in her 23rd year as a professional.
She has not won a Slam since the 2017 Australian Open, when she was eight weeks pregnant.
Williams says she still has the drive to win that elusive 24th title and believes she can still match Australian Court.
"I definitely do believe or I wouldn't be on tour," she said.
"I don't play just to have fun. To lose is really not fun."
Wang grew in confidence after she saved three break points in the sixth game of the opening set. Her firm forehands began to trouble Williams, whose movement could not match her opponent's.
Wang - who defeated current world number one Ashleigh Barty at the US Open before losing to Williams - broke for a 5-4 lead and then served out to seal the first set.
Williams was struggling to gain the upper hand as the unforced errors totted up - they would eventually reach 56 - and she was broken again when Wang converted the second of her three break points with a deep forehand winner.
The Chinese player's form deserted her as she served for the match at 5-4, with Williams breaking back with a superb forehand winner at the end of a 24-shot rally. The American saved two more break points at 5-5 before eventually levelling the match in a one-sided tie-break.
The final set went with serve until the 12th and final game. Wang had wasted two chances to seal victory on Williams' serve before she was presented with another, which was taken when the American netted a backhand.
"After the second set I was a little bit confused, but my mind always said I had to focus on the court, on every point and trust myself," said Wang, who now faces Tunisian Ons Jabeur in the fourth round.
Wang dedicated the victory to her former coach, Peter McNamara, who died from cancer just weeks before last year's US Open.
"I always dream about him," she said. "I think he can see what I play today. He will proud of me. I miss him."