Iga Swiatek of Poland serves to Zheng Qinwen of China during the United Cup tennis tournament in Perth, Australia, on Jan 3, 2024. (PHOTO / AP)

PERTH, Australia - World number one Iga Swiatek on Wednesday led top seed Poland into the semifinals of the mixed teams United Cup, while Serbia crashed out with an ailed Novak Djokovic suffering his first defeat in Australia since 2018.

Swiatek swept past Zheng Qinwen 6-2, 6-3 at Perth's RAC Arena to seal Poland's quarterfinal victory against China. After Hubert Hurkacz beat Zhang Zhizhen in the men's singles, Swiatek continued her ominous form ahead of the Australian Open and inflicted Zheng's first defeat of the tournament.

Swiatek was tested early against the rapidly rising Zheng, who had enjoyed eye-catching wins over Wimbledon champion Marketa Vondrousova and Serbia's Olga Danilovic.

Zheng Qinwen of China returns the ball to Iga Swiatek of Poland during the United Cup tennis tournament in Perth, Australia, on Jan 3, 2024. (PHOTO / XINHUA)

But she quickly gained the ascendancy as Poland became the first team to book a spot in the semifinals.

Hurkacz gave Poland the lead when he defeated Zhang 6-3, 6-4 in 82 minutes.

ALSO READ: Swiatek-led Poland, host Australia power into United Cup quarters

He outmuscled his opponent through dominant serving and furious groundstrokes to defeat Zhang, who lost his second straight match after starting with an impressive victory over Czech Jiri Lehecka.

Hubert Hurkacz of Poland stretches as he returns a shot to Zhang Zhizhen of China during the United Cup tennis tournament in Perth, Australia, on Jan 3, 2024. (PHOTO / AP)

Both teams used their reserve players for the dead rubber mixed doubles with Katarzyna Piter and Jan Zielinski wrapping up Poland's flawless performance with a 6-3, 5-7, 10-7 victory over You Xiaodi and Sun Fajing.

Host Australia reached the final four after brushing aside Serbia in front of a partisan crowd. Djokovic's extraordinary 43-match winning streak in Australia ended when he was stunned by 12th-ranked Alex de Minaur 6-4, 6-4.

It was the biggest win of de Minaur's career as he took advantage of Djokovic, who was troubled by his persistent wrist injury which affected him in his victory over Lehecka on Tuesday.

Zhang Zhizhen of China returns a shot to Hubert Hurkacz of Poland during the United Cup tennis tournament in Perth, Australia, on Jan 3, 2024. (PHOTO / XINHUA)

It's a worrying sign for Djokovic ahead of his bid for a record-extending 11th Australian Open title.

"It's extremely special. Novak is an unbelievable competitor, what he's done for the sport is incredible," de Minaur said. "It feels surreal, it feels amazing. This one definitely means a lot."

ALSO READ: Swiatek keeps winning in United Cup as Poland reach quarter-finals

Ajla Tomljanovic booked Australia's ticket to the semifinals by crushing Natalija Stevanovic 6-1, 6-1 in the women's singles.

In Sydney, Greece defeated Canada in both singles matches to book a quarterfinal clash with Germany.

Serbia's Novak Djokovic hits a return against Australia's Alex de Minaur during their men's singles match at the United Cup tennis tournament in Perth on Jan 3, 2024. (PHOTO / AFP)

Stefanos Tsitsipas started his season with a 6-2, 6-3 victory over Steven Diez in the men's singles. Tsitsipas allayed concern over his fitness after he withdrew from his match against Chile's Nicolas Jarry on Tuesday due to a back injury.

"Today was a good summary of how aggressive I can be," said Tsitsipas, who finished runner-up to Djokovic at last year's Australian Open. "I'm always seeking extra credibility in my game and whatever I can add to my game I am always willing and open-minded."

READ MORE: Poland through to United Cup semi-finals

Maria Sakkari sealed Greece's victory after downing Leylah Fernandez 7-6 (2), 6-3.

Australia's Alex de Minaur hits a return against Serbia's Novak Djokovic during their men's singles match at the United Cup tennis tournament in Perth on Jan 3, 2024. (PHOTO / AFP)

France booked a clash in the final-eight against Norway after clean sweeping Italy 3-0. Caroline Garcia had a tough 6-4, 5-7, 6-4 win over Italy's Jasmine Paolini after Adrian Mannarino's straight sets win over Lorenzo Sonego in the men's singles.

Elixane Lechemia and Edouard Roger-Vasselin tasted victory in the mixed doubles, as France topped Group D.