MELBOURNE (Reuters) - Serena Williams withdrew from the Yarra Valley Classic semi-finals on Friday due to a shoulder injury, just days before the American begins her quest for a 24th Grand Slam title at the Australian Open.
Williams had advanced to a mouth-watering semi-final clash against world number one Ash Barty after getting past Danielle Collins 6-2 4-6 (10-6) and while she showed no sign of discomfort the WTA Tour later said the 39-year-old was pulling out with a right shoulder injury.
Barty had also needed a super tiebreaker to beat Shelby Rogers 7-5 2-6 (10-4) in her quarter-final.
Super tiebreakers are being played after two sets in all WTA matches as organisers look to clear a backlog caused by Thursday's suspension of play after a worker at one of the Melbourne quarantine hotels contracted COVID-19.
In the other half of the Yarra Valley draw, Garbine Muguruza dumped Australian Open champion Sofia Kenin out of the tournament 6-2 6-2 in a re-match of last year's Grand Slam final at Melbourne Park.
Muguruza will next play Marketa Vondrousova after the Czech took down Nadia Podoroska 4-6 6-3 (10-4).
At the Gippsland Trophy, world number two Simona Halep suffered a surprise 6-2 6-1 defeat to Ekaterina Alexandrova.
Former Australian Open champion Victoria Azarenka advanced at the Grampians Trophy with a 6-4 1-6 (11-9) win over Yulia Putintseva but top seed Belinda Bencic fell 7-5 6-2 to Sorana Cirstea.
American Williams never looked in any real danger of following Kenin through the exit door despite a tricky tie against her compatriot, the 39-year-old ramping up her serve when it mattered to get the job done.
Williams, who had to pull out ahead of her second round match at Roland Garros last year due to injury, attended a media conference and spoke to reporters about her next match with Barty and there was no mention of any physical problem.
But her decision could be more out of caution with the year's first Grand Slam about to start from Monday.
"I'm really disappointed I can't finish out this #yarravallleywta tournament," Williams later said in a post on Instagram.
"Going to take a day off to get my shoulder ready for the @australianopen."
Barty and Rogers were among the first players out on court, playing under a closed roof at Margaret Court Arena on a rainy morning at Melbourne Park as the Australian Open warmups resumed following a COVID-19 scare.
"It was a tricky one, when the roof shuts here the conditions become quite sterile," said Barty.
"There were certainly some challenges today ... Happy to get through and get another opportunity tomorrow."
Two-time Grand Slam winner Halep made a brisk start to go up 2-0 against Alexandrova but the Russian then reeled off nine straight games as the frustrated top seed moved gingerly on court. She also put a heat pack on her lower back towards the close of the first set.
Ninth seed Alexandrova will next face Kaia Kanepi, who advanced after her quarter-final opponent Karolina Muchova withdrew from Friday's match due to an abdominal injury.
World number three Naomi Osaka eased past Irina-Camelia Begu of Romania 7-5 6-1 to set up a semi-final matchup against seventh seed Elise Mertens, who ousted third seed Elina Svitolina 6-3 5-7 (10-6) in the last quarter-final.
"I feel like I'm getting more comfortable as I play more matches, but I'm not sure about knocking off the rust," said three-time Grand Slam winner Osaka, who is playing her first tournament since last year's U.S. Open triumph.
"I feel like the nerves that I have for a Grand Slam are very different than the nerves that I have for a regular tournament."
(Reporting by Ian Ransom and Sudipto Ganguly in Mumbai; Editing by Peter Rutherford, Toby Davis and Christian Radnedge)