Serena Williams withdraws from Melbourne event ahead of semi-final clash with Ashleigh Barty

American 23-time Grand Slam champion cites a shoulder injury, days before start of Australian Open

Serena Williams walks off court after winning her quarter final match against Danielle Collins. Reuters

Serena Williams has withdrawn from the Yarra Valley Classic ahead of her semi-final match with world No 1 Ashleigh Barty due to a shoulder injury, the WTA announced.

Williams, a 23-time Grand Slam champion, had booked a showdown with home favourite Barty by beating fellow American Danielle Collins on Friday.

Williams, 39, had been in ominous form but needed to overcome a mid-match wobble to oust Collins 6-2, 4-6, 10-6 in 96 minutes. Hours after the win, the WTA revealed her withdrawal on Twitter, sending Barty straight through to the final.

"Serena Williams has withdrawn from the Yarra Valley Classic singles draw due to a right shoulder injury," the WTA wrote. "Ashleigh Barty advances to the final by walkover."

Williams is aiming for a record-equalling 24th Grand Slam title at the Australian Open and her withdrawal from the warm-up event is thought to be a precaution ahead of the start of the season's first major on Monday.

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

Storms forecast for Friday threatened to increase the backlog, 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 No 15 Garbine Muguruza has been in aggressive form in Melbourne, dropping just four games in two matches prior, and impressively hit the lines to run reigning Australian Open Sofia 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.

The two-time Grand Slam champion is showing the form that saw her rise to the top of the rankings in 2017 but it's a different story for defending Australian Open champion Kenin.

The 22-year-old's return to the scene of her stunning 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.

Muguruza will face Czech Republic's Marketa Vondrousova, who defeated Argentine Nadia Podoroska 4-6, 6-3, 10-4 in the last quarter-final.

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 Barty, 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.

Barty, playing with the roof shut on Margaret Court Arena, 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 in her season debut.