Japan's Naomi Osaka brutally halted Serena Williams's latest bid for a record-equalling 24th Grand Slam title on Thursday, as Novak Djokovic wrote the final chapter of qualifier Aslan Karatsev's fairy-tale run.

Fans returned after a five-day coronavirus lockdown in time to see an emotional exit from Williams, Jennifer Brady's win over Karolina Muchova and top-ranked Djokovic preserving his perfect semifinal record at Karatsev's expense.

Triple major-winner Osaka, 23, again proved Williams's nemesis with a one-sided 6-3, 6-4 win which heightened questions about whether the American, 39, will ever match Margaret Court's all-time record for Slam singles titles.

Williams put her hand on her heart as she acknowledged a standing ovation on Rod Laver Arena, and later walked out of 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 is into her first major decider.

But after going 0-2 down in the first set, Osaka reeled off five straight games to take charge against a despairing Williams and canter into the final.

The soft-spoken Osaka has never lost a Grand Slam match after reaching the quarter-finals, and Brady can expect the match of her life on Saturday.

"For me, I have this mentality that people don't remember the runners-up. You might, but the winner's name is the one that's engraved," Osaka said.

"I think I fight the hardest in the finals. I think that's where you sort of set yourself apart."

Williams lamented a "big error day" and refused to discuss whether she had made her last appearance at a tournament she has won seven times, an Open-era record.

"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."

Thousands of fans

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

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 also witnessed the end of 114th-ranked Karatsev's astonishing campaign, when he became the first man in the Open era to reach a Grand Slam semi-final on debut.

Karatsev knocked out three seeds to reach the last four but he was no match for Djokovic, the record eight-time Melbourne champion who has never lost a semi or final on his favourite court.

"This is the best I've felt in the entire tournament," said the Serb, who has been carrying an abdomen injury since the third round, after his 6-3, 6-4, 6-2 win.

"I could swing through the ball. No pain. The best match so far."

Exemplifying the gulf between them, Vladikavkaz native Karatsev, 27, has won 18 Grand Slam sets in his career, while Djokovic, 33, is going for his 18th major title.

Brady versus Muchova was a far more competitive affair as the American needed five match points before subduing the Czech 25th seed in three sets.

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."