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Williams eased past ninth-seed Johanna Konta 6-2 6-3 at Rod Laver Arena on Wednesday to move into a 10th consecutive grand slam semifinal and closer to a record 23rd major.
"It went pretty well for me today," Williams told reporters.
Williams will regain, too, the No. 1 ranking from German Angelique Kerber if she wins the tournament -- and might need to defeat older sister Venus to do so.
An all-Williams final is still a possibility, since Venus faces fellow American CoCo Vandeweghe on Thursday.
With the Williams sisters and Mirjana Lucic-Baroni in the last four in Melbourne, it means three of the four players in contention in the women's draw are at least 34. Vandeweghe, 25, is the exception.
Lucic-Baroni earlier beat one of the tournament favorites, fifth-seed Karolina Pliskova, 6-4 3-6 6-4, to set up a clash with the world No. 2 and make her second grand slam semifinal a remarkable 18 years after the first.
"No matter what happens, someone 34 or older will be in the final," Williams said.
Three years after making his first grand slam semifinal at Wimbledon, 15th-seed Grigor Dimitrov returned to the last four when the Bulgarian defeated No. 11 David Goffin 6-3 6-2 6-4.
Dimitrov -- who will play either 14-time grand slam winner Rafael Nadal or Wimbledon finalist Milos Raonic on Friday -- won his 10th straight match. Roger Federer challenges Stan Wawrinka in an all-Swiss tussle Thursday.
Williams will be the substantial favorite against the 79th-ranked Croatian, who fled to the US after she said she was abused by her father.
Faced with a difficult draw, the six-time Australian Open champion hasn't dropped a set in seeing off the likes of Belinda Bencic, Lucie Safarova -- two players who only dropped out of the top 10 due to health issues -- crafty Czech Barbora Strycova and Konta.
Britain's Konta -- a semifinalist here 12 months ago -- entered the event having won a warmup event in Sydney.
'Serve wasn't great'
Williams, however, wasn't overly pleased with her first serve, her most destructive shot.
She hit 10 aces and won 88% of her first-serve points but only served at 45%.
"My first serve wasn't really great," she said. "But I've really been working on my second serve."
Trailing by a break in the second at 3-2, the set turned when Williams recovered the break by chasing down a forehand down the line and a smash before racing forward to send a forehand winner down the line.
Despite the loss, Konta said facing Williams was probably one of the best experiences of her life.
"Credit to her, she played an almost perfect first set," Konta told reporters. "I felt she did incredibly well.
"She just showed and shows why she is who she is."