Ash Barty

Świątek took over the top ranking on Monday after Barty decided to call time on her career at the age of 25.

"I was crying for 40 minutes," Świątek told the BBC on Monday.

"Mainly, it was because of Ash's retirement. I didn't know it was going to happen and it really surprised me.

The integrity with which she carries herself and the historic career she is building before our eyes. But it still remained to be seen how she would handle and digest a moment like nothing before it.

“This whole situation has taken a lot of spotlight away from us competitors,” he said.

“We’re here to play the Australian Open. We’re here on our own terms, ready to compete and hopefully have a very good couple of weeks.”

But still, all anyone could talk about was the fate of the World No.1 and in official court documents, his lawyers warned it could have longstanding consequences.

“Cancelling Mr Djokovic’s visa would jeopardise the viability of Australia continuing to host the Australian Open,” they wrote.

Barty overcame a stumbling start to claim a 6-2 6-4 victory over Ekaterina Alexandrova at an empty Margaret Court Arena on Saturday.

The 24-year-old would have expected the backing of a fiercely partisan crowd for a weekend evening match at her home Grand Slam, but Melbourne's five-day lockdown left her playing in front of officials, ballkids and coaches.

While most other players have seen plenty of empty courts on the WTA tour over the last year, it was a new experience for Barty, who elected to stay home while the COVID-19 pandemic raged around the world.

Djokovic continued his imperious progress through the men's draw, negotiating a potentially tricky encounter with Diego Schwartzman 6-3 6-4 6-4 to reach the last eight at Melbourne Park for the 11th time.

The seven-times champion only added to a strong feeling around the tournament that the Serbian is again the man to beat at the year's first Grand Slam as he ousted the Argentine in just over two hours at Rod Laver Arena.

Federer, chasing a record-extending 21st Grand Slam, came from behind to beat Hungarian Marton Fucsovics 4-6 6-1 6-2 6-2.

Barty, ranked 223 in the world, scored a 7-5, 6-1 win over a player ranked 171 spots ahead of her.

"I've got goosebumps. This is amazing ... I'm shaking," the 20-year-old said.

In even better news for the Queenslander, she will face qualifier Mona Barthel for a spot in the round of 16 after the German beat Olympic gold medallist Monica Puig 6-4, 6-4.

Barty returned to tennis early last year after a stint playing professional cricket, with the Australian Open her first major campaign since the 2014 US Open.