Forty-seven players were barred from practising for a fortnight Saturday in a major setback to their Australian Open preparations after passengers on two charter flights that brought them to Melbourne tested positive to Covid-19.

Serena Williams came from a set down to oust 2017 champion Sloane Stephens 2-6, 6-2, 6-2, in what she called an "intense" encounter on Arthur Ashe Stadium.

The nine-team US circuit will stage its 45th campaign at The Greenbrier in White Sulphur Springs, West Virginia, from July 12-August 2.

Japan's Naomi Osaka retained the world number one spot.

Anastasia Pavlyuchenkova beat Sloane Stephens 6-7 (3/7), 6-3, 6-3 to set up a last eight clash with Danielle Collins.

Three-time Grand Slam winner Murray only played six tournaments last season and arrived in Australia early to acclimatize and prepare.

Svitolina looked sluggish after playing the two longest matches in the tournament against Wozniacki and Bertens in the semi-finals.

Sloane Stephens said on-court coaching created a connection between players and fans.

Six-time champion Serena Williams shook off a sluggish start to power past eighth-seeded Karolina Pliskova 6-4, 6-3 on Tuesday and into the US Open semi-finals.

Anastasija Sevastova will face either 23-time major champion Serena Williams or eighth-seeded Czech Karolina Pliskova.