Świątek in top form. Poland advances to United Cup semis

The top-seeded nation among the 18 that entered the $10 million event got off to a blazing start with big-hitting Hurkacz easing past Zhang Zhizhen 6-3 6-4.

Świątek then fought back from 2-0 down in the first set to beat Zheng Qinwen 6-2 6-3 and make it three successive singles victories in the event which the world number one is using to prepare for this month's Australian Open.

Poland will now travel to Sydney to play in the semi-finals, where they will be joined by Australia, who defeated Serbia in the day's other quarter-final clash in Perth.

Novak Djokovic crashed to his first defeat of the season and first in Australia for six years with a stunning 6-4 6-4 loss to Alex de Minaur who put Australia on course for a 2-0 win over Serbia.

"I knew that probably, (I was) not going to be at 100% physically, emotionally, mentally game-wise in the opening week of a season. Neither did I expect that," Djokovic, who had to have treatment on the wrist for the second day running, told a press conference.

"I was not on my level but it was just one of these days where I didn't feel my best on the court, and (my) opponent played very well."

Djokovic was hampered by a right wrist problem in Serbia's victory over Czech Republic on Tuesday.

However, the record 10-times Australian Open winner appeared optimistic about overcoming the wrist issue ahead of the first Grand Slam of the season.

"I think I'll be okay, to be honest," he said.

Caroline Garcia and Adrian Mannarino guided France into the quarter-finals with a 2-0 win over Italy. World number 20 Garcia beat Jasmine Paolini 6-4 5-7 6-4 after Mannarino beat Lorenzo Sonego 6-4 6-4 in the Group D tie.

Greece beat Canada 2-0 in Group B as Australian Open runner-up Stefanos Tsitsipas won his first singles match of the season beating Steven Diez 6-2 6-3 before Maria Sakkari rallied past Leylah Fernandez 7-6(2) 6-3.

The Australian Open runs from on Jan. 14-28.