A seismic day in the women's draw at the Australian Open saw Coco Gauff upset defending champion Naomi Osaka and Serena Williams lose to Wang Qiang.
Williams branded herself "unprofessional" after her bid for a record-equalling 24th Grand Slam singles title ended with a shock third-round loss to China's Wang.
When they met at the US Open last summer, Wang won just one game and 15 points, but it was a completely different story this time as the 27th seed claimed a 6-4 6-7 (2) 7-5 victory.
Williams won her first tournament in three years in Auckland earlier this month and arrived in Melbourne with a determined intensity that seemed to indicate she was ready to put four recent Slam final losses behind her.
But Williams made 56 unforced errors compared to 20 for her opponent, and said: "I just made far too many errors to be a professional athlete today.
"If we were just honest with ourselves, it's all on my shoulders. I lost that match. It's not about the tournament, it's just I can't play like that. I literally can't do that again. That's unprofessional. It's not cool."
Wang was on the brink serving for the match at 5-4 in the second set only for Williams' champions' mentality to kick in.
When she won the tie-break, it seemed tennis had read this script before and Williams would pull away to win, but instead Wang refused to be intimidated and kept her nose in front.
The Chinese player's nerves were obvious as she flunked her first two match points but Williams simply could not cut out the errors and a netted backhand sent her crashing out.
The American admitted she thought she had done the hard work by winning the second set, saying: "I was optimistic I would win. I thought, 'Okay, now finish this off'. I honestly didn't think I was going to lose."
Asked if defeats still sting as badly as they used to now she has so many other things in her life, Williams said: "I am just a better actress. I'm no happier than I was 10 years ago."
At 15, Gauff has now reached the fourth round at two Grand Slams, but this was on another level to any of her previous achievements as she comfortably defeated the player who lifted the trophy here 12 months ago.
There is a remarkable composure about the American and she kept her head impressively while Osaka lost hers, tumbling out of the tournament 6-3 6-4 in a flurry of errors.
"I love her, but I don't like this feeling of losing to her," said 22-year-old Osaka, who had beaten Gauff at the US Open last summer for the loss of just three games. "I think just losing to her, that hurts more than the defending champion thing. I think it's because I have an age problem. I don't like losing to people that are younger than me. I took this very personally."
Having been so composed during the match, Gauff was overwhelmed after it, saying: "Honestly, what is my life? Oh my gosh. Two years ago I lost the first round in juniors, and now I'm here. This is crazy.
"I was telling myself one point at a time and keep fighting because you never know what happens on this court. I'm on Rod Laver Arena, I can't believe this."
In the next round, the teenager will face another young American, 21-year-old 14th seed Sofia Kenin, who battled past Zhang Shuai 7-5 7-6 (7).
Top seed Ashleigh Barty eased into the last-16 with a 6-3 6-2 victory over in-form Kazakh Elena Rybakina and will next face Alison Riske, who she lost to at Wimbledon.
Petra Kvitova, the losing finalist 12 months ago, beat Ekaterina Alexandrova 6-1 6-2.