Serena Williams pulled out of a semi-final clash against world number one Ashleigh Barty with a shoulder injury Friday, days before the Australian Open, while Simona Halep and Sofia Kenin both crashed to defeat.

Williams, 39, had ousted fellow American Danielle Collins 6-2, 4-6, 10-6 in the Yarra Valley Classic but withdrew from the tournament hours later despite showing no ill effects during the match. 

The American, who is aiming for a record-equalling 24th Grand Slam title at the Australian Open, has looked in ominous form in Melbourne thus far.

Barty now has a bye into the final where she will meet either two-time Grand Slam winner Garbine Muguruza or Marketa Vondrousova.

World number three Naomi Osaka recovered from a shaky start to power past Irina-Camelia Begu 7-5, 6-1 in 76 minutes in the Gippsland Trophy, and will face fifth-ranked Elina Svitolina or Elise Mertens in the last four.

"She was a tricky opponent... I'm really relieved not to have to play a super-tiebreaker," said Osaka, who smashed 23 winners.

World number two Halep, however, slumped out after being thrashed by Ekaterina Alexandrova 6-2, 6-1 in the Gippsland Trophy, where she appeared hampered by a lower back injury.

Halep strapped on a back brace after calling for medical attention after the first set in a lopsided contest lasting just one hour.

Six build-up tournaments for next week's Australian Open returned to action Friday after a coronavirus case at a designated tournament hotel halted play Thursday and forced hundreds of players and officials into isolation.  

Happy Slam

Intermittent rain increased the backlog Friday, but to ease the congestion WTA clashes were reduced to two sets and a 10-point super tiebreaker instead of the traditional three sets.

World number 15 Muguruza has been in aggressive form in Melbourne, dropping just four games in two matches prior, and impressively hit the lines to run Australian Open champion Kenin ragged 6-2, 6-2.

"It's always better to know that you've played well here... and this is definitely the Happy Slam," said the Spaniard, using the Australian Open's nickname.

While she is showing the form that saw her rise to the top of the rankings in 2017, it's a different story for Kenin.      

The 22-year-old's return to the scene of her stunning Grand Slam breakthrough last year has been rocky, after playing just one set in her opener against Camila Giorgi, who retired hurt, and then surviving a three-set scare against fellow American Jessica Pegula.

Her struggles continued with 28 unforced errors as the flummoxed American departed in tears.

Earlier Barty, who has yet to rediscover her best after a long layoff, prevailed 7-5, 2-6, 10-4 in 92 minutes.

The 24-year-old, competing in her first tournament in 11 months, appeared fresh after Thursday's unexpected stoppage having played two singles matches within 20 hours earlier in the week.

She overcame a second-set stutter with a ruthless performance in the super tiebreaker.

Two-time Australian Open champion Victoria Azarenka saved two match points to beat Yulia Putintseva 6-4, 1-6, 11-9 and move into the quarter-finals of the Grampians Trophy, a third tournament being run concurrently in Melbourne, in her season debut.