Gao Xinyu continued her scintillating form at the United Cup on Monday, taking down Laura Siegemund in three sets as China secured a spot in the quarterfinals despite losing 2-1 to Germany.

The other winners on Day 4 were Poland, Britain and Kazakhstan, who will play Germany in the knockout round.

175th-ranked Gao has stolen the show at the mixed teams season-opening tournament, and her golden run continued at Perth's RAC Arena with a 6-1, 3-6, 6-3 upset of Siegemund in two hours and 17 minutes.

The result was enough to lift China into the knockout stage, finishing as the Perth group runners-up after Zhang Zhizhen and Zhang Shuai fell to Siegemund and Alexander Zverev 6-2, 7-6 (3) in the mixed doubles.

After bravely overcoming cramps to edge Brazil's Beatriz Haddad Maia in a marathon three-set match on Friday, Gao's confidence was evident as she stormed out of the blocks to overpower the 80th-ranked Siegemund in a one-sided first set.

Siegemund, however, rallied and forced a deciding third set before Gao regrouped, and she celebrated the gutsy victory by raising both her arms triumphantly in the air.

It was a must-win match for China after Zhang's earlier loss to world No. 2 Zverev, who overcame a sluggish start to prevail 2-6, 6-0, 6-2.

Zhang entered confident after losing just three games to Thiago Monteiro on Friday, and his clean striking from the baseline rattled Zverev.

The 45th-ranked Zhang also returned superbly to take the first set, but he was unable to sustain the intensity as Zverev gained control and prevailed in two hours and six minutes.

"I didn't start off well. I was missing more shots from my backhand side, which is normally the most stable," said Zverev, a two-time grand slam finalist. "He was also playing extremely well, so aggressive. He was not letting me into the match or find my rhythm."

"I'm happy with my performance in the second and third sets," he added.

All eyes in Sydney's late tie were on world No. 2 Iga Swiatek, who returned from her doping scandal in style with a crushing 6-1, 6-0 victory over Norway's Malene Helgo in 62 minutes.

The five-time Grand Slam champion looked sharp and dominated from the baseline in her comeback after a one-month suspension for testing positive for the banned substance trimetazidine.

"I'm happy for sure with the performance. It's not easy to play for your country. Sometimes the pressure is a bit bigger," Swiatek said.

World No. 6 Casper Ruud kept Norway's tournament hopes alive after beating Hubert Hurkacz 7-5, 6-3 in one hour and 23 minutes. The three-time grand slam finalist continued his winning start to the season, having beaten the Czech Republic's Tomas Machac on Sunday.

"Very happy with my performance. It was solid with a good game plan, and I was able to execute it well," he said.

But Norway was eliminated from the tournament after Ruud and Ulrikke Eikeri fell in the mixed doubles to Swiatek and Jan Zielinski 6-3, 0-6, 10-8. Poland and the Czech Republic will meet on Wednesday, with the winner securing a spot in the knockout stage.

Two-time grand slam finalist Stefanos Tsitsipas suffered a demoralizing straight sets defeat to 78th-ranked Alexander Shevchenko as Kazakhstan stunned Greece 3-0 in Perth.

World No. 11 Tsitsipas will be concerned about his form, having also needed three sets to beat Spain's Pablo Carreno Busta on Saturday.

He was thoroughly outplayed by Shevchenko and contributed to his demise with 40 unforced errors. The 26-year-old is clearly a long way short of the formidable figure who finished runner-up at the 2023 Australian Open.

It was a day to forget for highly-fancied Greece, with Maria Sakkari losing 6-4, 6-3 to Elena Rybakina in the women's singles.

"It was a tough match for me today, and I'm so happy to be in the quarterfinals," said Rybakina.

Earlier on Monday, Britain beat Argentina 2-1 after Katie Boulter and Charles Broom clinched victory with a 7-6 (4), 7-5 triumph over Maria Carle and Tomas Etcheverry in the deciding mixed doubles.

All three teams in Group F remain in tournament contention, with host nation Australia to play Britain on Wednesday.

Top-seeded United States will face Croatia on Tuesday, while Italy battles France.