Serena Williams withdraws from semi-final against Ashleigh Barty

Serena Williams hits a forehand at the Yarra Valley Classic — Serena Williams will begin her bid for a 24th Grand Slam singles title against Germany's Laura Siegemund at the Australian Open

Serena Williams has withdrawn from her Yarra Valley Classic semi-final against world number one Ashleigh Barty with a right shoulder injury.

Williams had earlier beaten fellow American Danielle Collins 6-2 4-6 10-6 in the last eight in Melbourne.

Australian Barty, who won 7-5 2-6 10-4 against American Shelby Rogers, has received a walkover into the final.

Williams will be seeking a record-equalling 24th Grand Slam singles title at the Australian Open.

The first Slam of the year begins on Monday and all six ATP and WTA warm-up events are being held at Melbourne Park.

Williams, 39, won her last major title at the Australian Open in 2017. She missed the 2018 competition and reached the quarter-finals in 2019, but fell in the third round last year.

Barty will play either Garbine Muguruza or Marketa Vondrousova in the final.

Spain's Muguruza thrashed Australian Open champion Sofia Kenin 6-2 6-2 in a rematch of last year's final.

America's Kenin made 28 unforced errors and left the court in tears after the 77-minute defeat.

Czech Vondrousova beat Nadia Podoroska of Argentina 3-6 6-3 10-4.

Third sets in the WTA are being played over a 10-point super tie-break following the delay to the tournaments because of Covid-19 testing.

Azarenka through but Halep out

Victoria Azarenka saved two match points in her first match of the year to beat Kazakhstan's Yulia Putintseva and reach the Grampians Trophy quarter-finals.

The Belarussian, who is a two-time Australian Open champion, overcame a dreadful second set to win 6-4 1-6 11-9.

She will play Estonia's Anett Kontaveit next after the sixth seed beat Bethanie Mattek-Sands 7-5 7-5.

Angelique Kerber, a former world number one and the 2016 champion at Melbourne Park, beat Ons Jabeur 6-4 6-4 to set up a quarter-final against Maria Sakkari.

However, top seed Simona Halep appeared to struggle with a lower back problem as she lost her Gippsland Trophy quarter-final.

Two-time Grand Slam champion Halep won the first two games of the match against Ekaterina Alexandrova before the Russian breezed past her 6-2 6-1.

Halep received medical attention after the first set and was given a lower back brace but her movement and power was lacking throughout the match.

Alexandrova will play Kaia Kanepi of Estonia in the semi-finals.

Japan's Naomi Osaka, who is one of the favourites for the Australian Open title, advanced with a 7-5 6-1 win over Irina-Camelia Begu of Romania.