Serena Williams broke down in tears and cut short her post-match press conference after another failed bid to equal Margaret Court's record Grand Slam haul, calling her defeat to Naomi Osaka "a big error day".

The veteran American's quest to reach a ninth Australian Open final and match Court's 24 major singles titles ended in a 6-3, 6-4 semi-final mauling by the Japanese third seed.

Williams, 39, broke early for a 2-0 lead before Osaka reeled off five straight games to leave her shellshocked.

"The difference today was errors. I made so many errors," said the 23-time Grand Slam champion. "Honestly, it was opportunities where I could have won. I could have been up 5-0. It was a big error day for me."

Pressed on what caused her to make so many mistakes, tears began to flow as she told reporters: "I don't know. I'm done," before walking out. In a clash of power games, it was Osaka who had the edge, hitting 20 winners to Williams's 12, while also committing three fewer unforced errors on 21.

Williams said she had been hitting well all tournament and had no explanation for the loss other than "too many mistakes there, easy mistakes".

Osaka's victory left her still stranded on 23 Grand Slam crowns, one short of Court's tally, with her last major title coming in Melbourne four years ago when she was pregnant. Williams has lost four Slam finals since and as she walked off court put her hand over her heart, almost as if saying farewell.

Asked about it afterwards, she suggested the gesture was more about acknowledging the crowd, who were allowed back into Rod Laver Arena after a five-day snap lockdown and gave the American a rousing send-off. "I don't know. If I ever say farewell, I wouldn't tell anyone," she said

But Williams later posted a heartfelt message to her Australian fans on Instagram. "Today was not ideal outcome or performance but it happens ... I am so honoured to be able to play in front of you all," she said. "Your support -- your cheers, I only wish I could have done better for you today. I am forever in debt and grateful to each and everyone single one of you. I love you. I love you. I love you. I adore you."

Osaka, who is 16 years younger than Williams and grew up watching her, said she was honoured to be on the same court.

"I mean, it definitely means a lot. I think, of course, every time I play her, I feel like it's something I'll definitely remember a lot," she said, adding that talk of the American retiring made her "sad". "I want her to play forever," she said. "That's the little kid in me."