The 23-time Grand Slam champion usually doesn't play in a tournament in the week before a Grand Slam event, but made an exception before last year's U.S. Open and for this Australian Open because of all the disruptions caused by the COVID-19 pandemic.
The 39-year-old American was playing her first tournament since withdrawing from the French Open before her second-round match because of an Achilles tendon injury in October. She, like all players traveling to Australia for the tournament, had to quarantine for 14 days and had limited practice during that time.
With the year's first major starting Monday, Williams may be better resting.
She had three wins at the Yarra Valley Classic — one of six tournaments at Melbourne Park this week — culminating with a 6-2, 4-6, 10-6 win in a match tiebreaker over Danielle Collins on Friday.
The withdrawal came after her post-match news conference — when she made no reference to any issues — so no full explanation was offered.
It means Barty gets a walkover into the final and will also get some rest Saturday.
There were 70 matches on the order of play Friday, an expanded program a day after all matches in six tournaments were postponed so that 160 players and 507 people overall could isolate and undergo testing because a worker at a quarantine hotel returned a positive test. Tennis Australia said all tests were negative.
To speed things up a little at the back end of the tournament, 17 courts were being used and women's matches were reduced to two regular sets with a 10-point match tiebreaker if required.
Williams and Barty both had to play the match tiebreaker — Barty beat Shelby Rogers 7-5, 2-6, 10-4.
Before withdrawing, Williams said she was looking forward to the semifinals.
"It's definitely a good opportunity to see where I'm going against the current No. 1, that's really important for me," Williams said. "Obviously I think for her, as well, to see where she's going against my game.
"It's been a long 12 months. Tennis has been played but not really, so ... hopefully this is like a new year where things will be different."
Garbine Muguruza beat defending Australian Open champion Sofia Kenin 6-2, 6-2 to advance to the semifinals, reversing their result of their 2020 final at Melbourne Park. She will next play No. 8 Marketa Vondrousova.
OSAKA IN, HALEP OUT
U.S. Open champion Naomi Osaka moved into the semifinals of the Gippsland Trophy by beating Irina Begu 7-5, 6-1.
That made it two losses from two for Romanian players in the quarterfinals. Top-seeded Simona Halep lost 11 of the last 12 games in an upset 6-2, 6-1 loss to No. 9 Ekaterina Alexandrova.
Alexandrova will next play Kaia Kanepi, who got a walkover against No. 8 Karolina Muchova.
No. 7 Elise Mertens was unable to close out on serve in the second set but recovered to beat No. 3 Elina Svitolina 6-3, 5-7, 10-6 in the last match on Margaret Court Arena. She'll play Osaka in the semifinals.
AZARENKA ADVANCES
Two-time Australian Open champion Victoria Azarenka rallied from two breaks down in the first set and had to save two match points in the match tiebreaker before beating Yulia Putintseva 6-4, 1-6, 11-9 to advance at the Grampians Trophy, an event staged for the players forced into hard lockdown during quarantine.
On either side of a delay on outside courts because of rain, former Wimbledon, U.S. Open and Australian champion Angelique Kerber beat No. 8 Ons Jabeur 6-4, 6-4 and Belinda Bencic, the highest-ranked player in the tournament, lost to Sorana Cirstea 7-5, 6-2.
GERMANY UPSETS SERBIA
Alexander Zverev rebounded from a narrow singles loss to Novak Djokovic to help Germany win the deciding doubles match that earned the team a spot in the ATP Cup semifinals at the expense of defending champion Serbia.
Jan-Lennard Struff beat Dusan Lajovic 3-6, 6-3, 6-4 to give Germany a 1-0 lead in Group A, but the top-ranked Djokovic leveled it up with a 6-7 (3), 6-2, 7-5 win over Zverev.
Struff and Zverev combined to beat Nikola Cacic and Djokovic 7-6 (4), 5-7, 10-7 in the doubles, advancing to the semifinals against Russia on Saturday.
"It's unfortunate we lost the tie — that's what this competition is all about," Djokovic said of back-to-back matches with Zverev. "It's about the team, not about winning one match, singles match. Yeah, it's a tough one."
Pablo Carreno Busta clinched a semifinal spot for Spain with a 6-3, 6-4 win over Michail Pervolarakis in the opening singles match against Greece, even though the Greeks went on to win 2-1. Spain ended up with enough points to top Group B, which also includes Australia.
"I knew we needed this point," said Carreno Busta, who was playing again because second-ranked Rafael Nadal had back stiffness. "With this victory, we are into the semifinals, so it was very important to us."
Stefanos Tsitsipas later beat Roberto Bautista Agut 7-5, 7-5 and then combined with Pervolarakis in the deciding doubles match, which lasted only one game before the Spanish pair retired.
WAWRINKA, DIMITROV OUT
Having to play rounds back-to-back took a toll on the top-seeded players at the Murray River Open, with No. 1 Stan Wawrinka, No. 2 Grigor Dimitrov and No. 4 Borna Coric out in the quarterfinals after wins over Australian players earlier in the day.
Wawrinka withdraw from his quarterfinal match against Jeremy Chardy, citing fatigue, after his third-round win over Alex Bolt took almost 2 1/2 hours. Dimitrov lost his to quarterfinal match to Corentin Moutet 7-5, 6-2 after his earlier three-setter against Alexei Popyrin.
Daniel Evans completed two wins in a day to reach the semifinals. He had a quarterfinal win over Coric, who was coming off a win over Nick Kyrgios.
No. 3 Felix Auger-Aliassime advanced to the semifinals when Jiri Vesely retired after losing the first set 7-6 (3).