Poland’s Iga Swiatek (bottom left) and Hubert Hurkacz (bottom right) hit a return against Spain’s Sara Sorribes Tormo and Alejandro Davidovich Fokina during their mixed doubles match at the United Cup tennis tournament in Perth on Jan 1, 2024. (PHOTO / AFP)

PERTH, Australia - World number one Iga Swiatek was in sublime form on Monday to power Poland into the United Cup quarterfinals, while host Australia also progressed.

Swiatek was under pressure when she hit the court in Perth after Alejandro Davidovich Fokina gave Spain the lead with a comeback 3-6, 6-3, 6-4 victory over world number nine Hubert Hurkacz.

But Swiatek, winner of four Grand Slam titles, was undaunted and swept past Sara Sorribes Tormo 6-2, 6-1. The 22-year-old Swiatek has only lost seven games in two singles matches as she starts her preparation for the Australian Open in style.

"I'm really happy with my level and the way I'm focused," Swiatek said. "I'm not feeling rusty. I'm not feeling like I need to get into a lot of rhythm."

Swiatek then combined with Hurkacz to thrash Sorribes Tormo and Davidovich Fokina 6-0, 6-0 in the mixed doubles as Poland awaits a quarterfinal opponent.

After losing their opening tie against Britain, Australia faced a must-win clash against title holders United States in Group C.

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Alex de Minaur of Australia hits a return to Taylor Fritz of the United States during the United Cup tennis tournament in Perth, Australia, on Jan 1, 2024. (PHOTO / AP)

Alex de Minaur kept alive Australia's quarterfinals prospects by defeating Taylor Fritz 6-4, 6-2 in an eye-catching performance.

"I was in a great mindset and I played the type of tennis I wanted to be played," said de Minaur, who was sluggish in his opening loss to Britain's Cameron Norrie.

Jessica Pegula overcame a patchy performance to defeat Ajla Tomljanovic 7-6 (6), 6-3 and level the tie. The American had been stunned by Britain's Katie Boulter in her opener, but started rediscovering her top form to run over 292nd-ranked Tomljanovic.

It came down to the mixed doubles with the United States only needing to win a single set to qualify.

But Australia's Matthew Ebden and Storm Hunter beat Pegula and Rajeev Ram 6-3, 6-1 much to the delight of the parochial crowd at RAC Arena.

In Sydney, France spoiled Germany's bid to finish top of Group D with a thrilling 2-1 victory.

Alexander Zverev had given Germany the lead after a 4-6, 6-4, 6-3 fighting victory over Adrian Mannarino in his ninth straight win over the Frenchman.

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France's mixed doubles team of Caroline Garcia (right) and Edouard Roger-Vasselin play against Germany during the United Cup tennis tournament in Sydney on Jan 1, 2024. (PHOTO / AP)

Angelique Kerber looked set to clinch Germany's quarterfinal spot until Caroline Garcia stormed back with a 1-6, 6-2, 6-2 victory to force a decisive mixed doubles.

Garcia and Edouard Roger-Vasselin then prevailed 7-6 (4), 2-6, 12-10 in a titanic battle over Zverev and Kerber. France can book their berth in the quarterfinals by beating Italy on Wednesday.

In the earlier tie, the 11th-ranked Casper Ruud led Norway to a 2-1 victory over Croatia. He was under pressure after Croatia made a winning start in the Group F tie with Donna Vekic overcoming Malene Helgo 7-5, 3-6, 6-3.

But Ruud, who reached a career high ranking of number two in 2022, won his second straight singles match to start the season after beating Borna Coric 6-4, 6-1 in just 89 minutes.

He then partnered with Ulrikke Eikeri to beat Vekic and Ivan Dodig 6-2, 3-6, 10-7.

World number one Novak Djokovic will return to action on day five as he spearheads Serbia's push for the quarterfinals when they face the Czech Republic in the only tie in Perth.

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Stefanos Tsitsipas starts his season when Greece takes on Chile, while Croatia and the Netherlands meet in the later tie in Sydney.