Elena Rybakina's Kazakhstan dumped defending champions Germany out of the United Cup with Alexander Zverev sidelined by an arm injury barely a week away from the Australian Open, while top seeds USA beat China to reach the semi-finals.

In Sydney, women's world number two Iga Swiatek led Poland into the last eight by winning a rematch of her 2023 French Open final against the Czech Republic's Karolina Muchova.

Britain also progressed to the quarter-finals with Katie Boulter's dominant 6-2, 6-1 victory over Australia's Olivia Gadecki enough to guarantee they won their group.

The upset in Perth sent the Kazakhs into the semi-finals of the 18-nation tournament.

Sixth-ranked Rybakina earned the first point for Kazakhstan, with the 2022 Wimbledon champion hammering Laura Siegemund 6-3, 6-1.

Zverev then withdrew from his clash against Alexander Shevchenko at the last minute with a bicep strain, leaving the 254th-ranked Daniel Masur to face him.

He put up a brave fight but Shevchenko was too strong, powering home 6-7 (5/7), 6-2, 6-2 despite a medical timeout in the second set, complaining about difficulty breathing.

"I was struggling in this heat, it was so rough," said the world number 78, who upset Stefanos Tsitsipas in group play on Monday.

"I had a headache and my head was spinning, but I wanted to win this one." added Shevchenko. "After [losing] the first set, I knew I had to shorten the rallies and play aggressively."

In the other quarter-final, 2023 winners United States beat China thanks to singles rubber wins by Coco Gauff and Taylor Fritz.

World number three Gauff beat Zhang Shuai 7-6 (7/4), 6-2, and Fritz then put the tie beyond the Chinese with a 6-4, 6-4 defeat of Zhang Zhizhen.

"Coco did such a good job putting us ahead and giving me the chance to come out and put us through," said Fritz.

"There was a little bit of pressure because I didn't want to be the one who loses it for the team after Coco won all her matches this week," he added. "I played a really solid match – really happy with it." (AFP)