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.
Meanwhile, former world number one Caroline Wozniacki saw her hugely impressive career come to a close as she went out in the Australian Open third round.
Wozniacki, 29, revealed her intention to retire in November, fittingly calling time at the venue where she won her only Grand Slam title.
Now ranked 36th, Wozniacki's 15-year career ended with a 7-5 3-6 7-5 defeat by Tunisia's Ons Jabeur.
"It has been a great ride," said the emotional Dane on Melbourne Arena.
"It is fitting my last match would be a three-setter, a grinder, and I'd finish my career with a forehand error - those are the things I have been working on my whole career," joked Wozniacki.
"I guess it was meant to be."
Wozniacki bows out having spent 71 weeks as the world number one, lifted 30 WTA titles and accumulated almost £27m of prize money.