Japan's Naomi Osaka brutally halted Serena Williams's latest bid for a record-equalling 24th Grand Slam title as she set up an Australian Open final with Jennifer Brady on Thursday.

Three-time major-winner Osaka once again proved the American's nemesis with a one-sided 6-3, 6-4 win which heightened questions about whether Williams, 39, will ever match Margaret Court's all-time record.

Williams put her hand on her heart as she acknowledged a standing ovation on Rod Laver Arena, and later cut short her post-match press conference in tears, saying: "I'm done."

The American has lost four Grand Slam finals since winning her 23rd in Melbourne in 2017, and victory over Osaka would have given her a golden opportunity against Brady, who will be playing first major decider.

But US Open champion Osaka has never lost a Slam semi-final and after going 0-2 down in the first set, she reeled off five straight games to take charge against a despairing Williams.

"I hit a lot of unforced errors in the first few games, I was just really nervous and scared," said Osaka. "But then I sort of eased my way into it and the biggest thing for me was having fun."

Williams lamented a "big error day" and refused to discuss whether it might have been her last Australian Open. "I don't know," she said, when asked if her salute to the crowd was a farewell. "If I ever say farewell, I wouldn't tell anyone."

Williams's exit was watched by thousands of fans returning after a state-wide snap coronavirus lockdown, a welcome sight in Rod Laver Arena after five days of matches in front of empty seats.

'My legs are shaking'

The Australian Open is the first Covid-era Grand Slam to welcome large numbers of fans after last year's Wimbledon was cancelled, the US Open took place behind closed doors and the French Open was limited to 1,000 a day.

The returning spectators then witnessed more fireworks in Brady's victory over Karolina Muchova, where the American needed five match points before subduing the Czech 25th seed.

America's Brady eventually prevailed 6-4, 3-6, 6-4 in 1hr 55min to reach her debut major final against the formidable Osaka.

"My legs are shaking, my heart is racing," said Brady, 25, who has admitted she hated watching and playing tennis as a child. "My legs felt fresh but they weren't moving. They felt stuck in mud."

In the night match, world number one Novak Djokovic faces an unlikely foe in the men's semi-finals -- Russian qualifier Aslan Karatsev, who is enjoying one of the greatest Grand Slam debuts in history.

Exemplifying the gulf between them, Karatsev, 27, has won 18 Grand Slam sets in his career, while Djokovic, 33, is going for his 18th major title. Vladikavkaz native Karatsev is bidding to become the first player to reach the final on his Grand Slam debut, in just his 19th Tour-level match.

"I try to believe what I'm doing on the court, and it's helped me to win matches," said Karatsev. "I arrive on the court and say, 'Okay, I try to do my best with more belief.'

Djokovic, who is carrying an abdominal injury, has won 13 of his last 14 Grand Slam semi-finals, and is going for a record-extending ninth Australian Open crown. "He doesn't have anything to lose, really," said the Serb of his Russian rival. "He's motivated."